Ok, first, if my children's DR told me to give them Miralax at such a young age, I'd find a new DR!!! There are so many other safer things you can do for him than that stuff.
Have you tried giving him 1/4 to 1/2 a tsp (1 time a day in a bottle) of light karyo syrup? I know some DRs have said to give dark but I've only ever used light for my kids. Also, fleets makes children's suppositories. You can break one in half and use that to help him go. Prune juice a few times a week.
I have no experience with Similac except for what my friends have dealt with and it has caused the same problems with their kids too. Gassy, hard stools, not pooping very much.....Enfamil did the same thing to my daughter too. My son though was on Carnation Good start and he pooped like he almost breast fed on that stuff!! He was never gassy or had problems going. Wic will change your formula to another brand but your Dr has to write you a prescription for it and you'll take it to Wic.
Or get one can of the powered our of pocket and try it first to see how he handles it. Then if you like it, you can go back to his Dr and tell him you want him switched over to it.
Good luck!
S.