There's quite a few that would fit.
He's bitten off more than he can chew.
to try to do more than you are able to do Don't bite off more than you can chew.
Let someone else organize the party.
bite off more than can chew.
(n.d.)
Cambridge Idioms Dictionary, 2nd ed..
(2006).
Retrieved October 10 2014 from http://idioms.thefreedictionary.com/bite+off+more+than+can+chew
He's got too much on his plate.
Fig. to be too busy.
I'm sorry
, I just have too much on my plate right now.
If you have too much on your plate, can I help?
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
S.v.
"have too much on plate.
"
Retrieved October 10 2014 from http://idioms.thefreedictionary.com/have+too+much+on+plate
He's got too many irons in the fire.
Fig. to be doing too many things at once.
Tom had too many irons in the fire and missed some important deadlines.
It's better if you don't have too many irons in the fire.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
S.v.
"have too many irons in the fire."
Retrieved October 10 2014 from http://idioms.thefreedictionary.com/have+too+many+irons+in+the+fire
ell.stackexchange.com/users/6494/Frank
One idiom relates to hats.
When someone has two jobs, we sometimes say they have two hats: When I'm wearing my programmer hat, I'm a programmer.
When I'm wearing my musician hat, I'm a musician.
Macmillan Dictionary defines it this way and gives the following examples: (informal) one particular aspect of someone's duties or responsibilities
She has to wear several different hats
(=have various responsibilities) in her position.
Of course, when I say this, I'm wearing my teaching hat.
So you could say he's wearing too many hats .
ell.stackexchange.com/users/230/snailplane
He has too many balls in the air, see: http://en.wiktionary.org/wiki/too_many_balls_in_the_air
ell.stackexchange.com/users/10991/jahoo
Another for you: "He's in over his head."
ell.stackexchange.com/users/5118/njahnke
You could also say that somebody is " spinning too many plates " or "trying to spin too many plates at once".
ell.stackexchange.com/users/1532/JMK
If you can imagine someone trying to put up wallpaper with only one arm/hand, you'd say "Busier than a one-armed paper hanger".
Or there's always "Busier than a one-legged man in an a**-kickin' contest!"
Or that old-time favorite "Busier than a cheap whore when the fleet's in!" Share and enjoy.
ell.stackexchange.com/users/4943/Bob Jarvis - Reinstate Monica
Something like "His head is in ten different places".
I need a proverb or idiom which can roughly mean "He has taken up more jobs than he can handle"
ell.stackexchange.com
<idioms><proverbs>
ell.stackexchange.com/questions/35389
ell.stackexchange.com/users/4721/Sandeep D
When someone has two jobs, we sometimes say they have two hats, so you could say he's  wearing too many hats. Alternatively, "he's got too many irons in the fire", "trying to spin too many plates at once", "he's got too much on his plate", "he's bitten off more than he can chew" or "he's in over his head." Too many hats, too many irons in the fire, too many plates, too much on their plate are all options. Biting off more than they can chew, in over their head are possible.
Too many hats, too many irons in the fire, too many plates, too much on their plate are all options. Biting off more than they can chew, in over their head are possible.