We had a guy like that when I was in boot camp...except he was 5'9" and maybe 120! The drill seargeants stayed on his ass constantly, dropped him for whatever pushups he could manage whenever he looked sideways, loaded his plate with food everytime he went into the mess hall, and stood over him and made him eat it all.

By the time we finished basic, he was able to complete the physical challenge, and knocked out 30 pushups! And that was in just 6 weeks!

Point is, it can be done, although sometimes it might take a personal trainer for some folks!

Best of Luck,
Jeff