Has anyone had the experience of a pup that was a lot smaller than its litter mates and how they grew as an adult? Our pup was fairly ill when he was younger and we saw several of his litter last weekend and he is even smaller than the bitches. Will he make up the difference in time (he is 7 months old), or will he always remain smaller?

It could go either way.  He may just take longer to reach his full size, or his setback may have permanenly set him back, only time will tell, but at 7 months he has plenty of time to grow on.