I skimmed many of the answers. I hope I'm not repeating. My thought is that he may need some play time before homework. Maybe sitting still and following directions all day in school is wearing him out and he needs a break before homework. Could always be the other way around: sit down and get the homework done and out of the way before play. Try some different routines to see if one works better than the other.
Also, I would definitely talk to the teacher to see what she (or he) would suggest and to get input on how to handle things when he just doesn't seem to be getting the homework. Communicating with the teacher about what homework he struggles with helps the teacher to know what he needs help with.