Hi S.,
Sorry to hear that your little man is having trouble. It can be very concerning when they don't have bowel movement for a while. It sounds like you are doing all of the right things, one thing that can really help is water, try giving him 4oz of warm water, if he will take it try 8oz spread through out the day, give him a little while then try massaging his tummy, puch his legs up to his tummy and move them round in a bicycle movement, all of these combined should help him to relax and get them bowels moving again. Keep with the water everyday, the switch from the breast milk to formula may just be taking his digestive system a while to get used to. The formula doesn;t contine as much 'water' as breast milk.
If you try this and he is still struggling then, def go back to your ped tell them everything you are trying and have done. They may advise a change of formula to a partially broken down one such as Nutramagem (my daughter was on this until 12 months as she had the same issues you describe)
The last thing your ped may suggest is Mirilax, which I actually still have to give my daughter now and she is just 2, she has a qtr to a half tsp everyday and certain food items still bung her up. Until her digestive system is fully matured, the dr told us to watch her dairy intake and no bananas!
Good luck huni.