You use a manual chair? If so you need a snuggly or if that is impossible to use independently I'd rig up a seatbelt that ties around the both of you. I sewed a strap to the back of a pair of overalls and a vest that my wheelchair seatbelt would go through. Kinda like this
http://www.babybair.com/product.html . Also a stroller or highchair on wheels.
Ummm....I guess you've got diaper changing figured out? I have a wall mounted change table like the ones in public restrooms but can change on a bed or table. Where does he sleep? We modified a crib but both my kids slept in their bassinet until they were nearly 6 months. With picking them up I hook my wrists under their armpits and make sure theres always somewhere I can push myself back up with my elbow.
Anything in particular just ask...