An overweight cat that isn't eating properly has a high risk of developing fatty liver disease, so I would be inclined to take him back to the vet for a follow-up exam and blood work. He may not be pooping much because he isn't eating much, or it could also be the other way around (constipation may cause him not to want to eat). In the meantime, offer him a wide variety of things to try to spark his appetite: low sodium canned salmon (with the juice for extra moisture), plain meat baby food (no onions), a bit of plain unsweetened yogurt, Temptations cat treats, broiled chicken breast, stinky canned Fancy Feast (fish flavours are the smelliest), whatever it takes to get him eating. Here is more info on dealing with anorexia in cats: http://www.assistfeed.com/FelineAnorexia.htm
For the fleas, get a flea comb and start combing him a couple times a day. Dip the comb in soapy water after each pass thru his fur. Make sure to wash all his bedding and yup, lots of vacuuming. Good luck!