Like Matt Brennan in the comments above, I'd also consider a)
"He always eats like a pig." more natural than your b) "
He is always eating like a pig."
In fact, these two expressions have a subtle difference in meaning.
Specifically, the word "always" generally means "every time" when applied to the simple present tense as in (a), but "all the time" when applied to the continuous tense in (b).
Thus, sentence (a) is simply a statement about the subject's consistently bad eating habits, whereas sentence (b) also implies that he spends a lot of his time eating, which may or may not be what you want to imply.
(In fact, you could even leave out the word "always" from sentence (a) entirely — a simple present sentence that does not, by itself or in context, refer to any specific time is generally understood to describe habitual behavior.
Thus, simply saying "he eats like a pig" is perfectly sufficient to describe someone's typical eating habits.)
Also, as noted by hunter and queeg, the idiom "eat like a pig" carries connotations of both eating messily (which you seem to want) and eating a lot (which you apparently don't).
There is a related idiom, "eat like a horse" , which implies only the latter, but
no common idiom that I'd know of that would only suggest messy eating habits without implying anything about the quantity eaten.
That said, you can certainly clarify the meaning with a subordinate clause, as you've done in your original sentence.
Thus, the following sentence would work perfectly well: a') "
He (always) eats like a pig, scattering crumbs all over the table."
A minor remaining dissonance is that, at least to my ear, "eating like a pig" would usually imply a somewhat higher degree of messiness than merely leaving crumbs on the table — the mental image I get is of someone eating as much and as fast as they can, making a lot of noise and splattering food (not just crumbs) all over the place.
Using such a strong idiom, and then implying that the real issue is just a few crumbs on the table, makes the speaker sound very fussy — which, of course, could be exactly the effect you're going for.
If not, though, you might want to go for something a bit less evocative, like, say, simply "he always eats messily, [...]."
ell.stackexchange.com/users/498/Ilmari Karonen
The verb tense is executed exactly correctly (for just the reason that you say); however, "Eat like a pig" might be understood to mean "eat a lot," as opposed to "eat in a sloppy fashion."
(But it's not wrong to use it the way you did either.)
You mean "crumbs," not "crumbles."
ell.stackexchange.com/users/3488/hunter
There are at least four distinct possibilities here: "He always eats like a pig" means that, when he eats, he always does it messily and/or to excess; "
He is always eating like a pig
" means that he eats all the time and, furthermore, he eats messily and/or to excess; "He is always eating, like a pig" means pigs eat all the time and so does he; "
He always eats, like a pig" means that pigs eat whenever they have the opportunity and so does he.
ell.stackexchange.com/users/4468/David Richerby
If you change 'crumbles' to 'crumbs' the sentence would be ok.
Crumbles is what happens, for example, to a dry biscuit that you break and crush.
Crumbs are the mess left behind after eating said biscuit, usually down the front of your clothes.
If, whilst eating, I'm gulping big mouthfuls of food, barely chewing, gasping for breath, eating all that is in front of me, focussing on little else (imagine smudges of food over my face), then saying I'm 'eating like a pig' would be apt.
Think of Mr Creosote from the Meaning of Life.
Pigs eat anything and everything (pigfarms in Snatch).
But if I'm making a mess with crumbs (crumbsies on his jacketsies - Gollum to Sam)
, I'm unlikely to be called a pig.
But I could be called mucky, careless, scruffy.
Another animal 'idiom' would be a muckypup.
I'd be quite offended if someone called me a pig for getting digestives/elvin bread/wafer thin mint down my front.
But I'd probably smile (if I heard) if I was compared to a pig whilst my face was buried into a mountain of food.
ell.stackexchange.com/users/10515/queeg
(I don't have high enough reputation to comment, otherwise this would be a comment.)
'
He is always eating like a pig...'
does flow a little oddly, but I'd use a contraction, i.e. 'He's always eating like a pig...' :-) '
He always eats like a pig' flows a little oddly to my ear, personally.
I'm a native Australian English speaker, though it's not my first.
ell.stackexchange.com/users/10920/janneia
Though not a native English, I was taught that always + continuous tense can express a strong dislike or aversion towards the act mentioned.
ell.stackexchange.com/users/6788/András Hummer
Does it sound right to say:   He is always eating like a pig, leaving crumbs all over the  table.   (Using Present Continuous, because of always + negative connotation)
Idiom "to eat like a pig"
ell.stackexchange.com
<word-usage><idioms>
ell.stackexchange.com/questions/34979
ell.stackexchange.com/users/5754/Denis Kulagin
 The verb tense is executed exactly correctly (for just the reason that you say); however,   "Eat like a pig" might be understood to mean "eat a lot," as opposed to "eat in a sloppy fashion." This can be understood as eating a lot as well as eating sloppily.
This can be understood as eating a lot as well as eating sloppily.