Hello,
My son (now 6 yrs old) was switched to Nutramigen when he was small due to his SEVERE colic. It worked wonders! I kept him on that until he was off the bottle (at 1 yr old) - we switched him to whole milk & he HATED IT - but our dr. said water & juices are fine as long as he's getting his vitamin d from other sources (which he was) and eventually when he turned about 3 he'd finally drink milk & loves it now at 6.
If I were you, again only just my opinion, I'd keep him on it until you're ready to get him off bottles all together. We took bottles away by 1 yr old. and with the bottles went the formula. Definitely don't switch back anytime soon - the special formula is working - continue to use it!
My son ate all his baby food (stage 1 & 2) while on the formula & had no problems at all what-so-ever. The formula is just easier to digest for babies, so it won't get all 'chemistry set' on your son when adding food to his diet.
Good luck and best wishes
from another SAHM of 2 boys (6 & 3.5 yr old)