First of all sex with an "overweight" man is vague.
Does he weigh 400 lbs or is 250? Both are overweight.
Secondly. How large is his pannus? [the roll of fat that hands down below the waist]. Is it so large that it inteferes with the contact between you too?
How large are you?
The first thing I would suggest is more positions with you on top. Its jus a natural progression when your not working with much.

also try positions where your doing the "splits" on top of him.
Also there is an option of him trying a penis pump. I have one and it does work. You will here claims it doesnt work, its phony but for 100 or so you can get a real pump that will give him length. I know because I [am/was] small below the belt. The gains arnt permanet, but 2 extra inches while you plan to have sex is alot. He will also gain gerth. Email me if oyu want.