Reintroducing Cows Milk Formula

So my son had suspected Cows milk allergy. My daughter who is now 18 months was the same so we were aware of signs and symptoms. He was first put onto Nutrimigen but unlike my daughter still struggled so I asked to be swapped to Neocate. He's been a lot better but is still a very fussy boy. I've read some things about Neocate and the side effects, and now that Albie is older I want to see if it really was an allergy and if he will tolerate normal formula. I have been introducing gradually. First bottle of the day having one scoop of normal and 6 of the usual, Day 2 having 2 scoops of normal and 5 of the usual and so on. Until Day 7 requires the first bottle being all normal formula. So far he's had no rash, not being extremely hot after a bottle which are the signs when he was first diagnosed. Now albies normally quite constipated on neocate. His poops and farts have a distinct smell. I noticed after the first bottle and second that it's stopping him being constipated but his poops are more runny. I'm not sure if this is a reaction or not. He's happy in himself and there are no other signs. Could it be the normal Cows milk is just getting his tummy working better than neocate? I'm going to keep going until a full bottle but I just don't know if it's helping or not