Sounds like reflux, especially with the crying right after eating and overeating. And reflux can definitely cause congestion.
As far as the constipation, if he has small pellet or hard balls instead of soft or liquidy stool, then you should definitely talk to his pediatrician about this. Babies this young are rarely truly constipated (small hard stools) and if they are they should be evaluated by a doctor. But babies this age do often go days at a time without pooping (which is normal) and may have some discomfort as they adjust to this new feeling...but I bet the misery your baby has is more from the reflux than from the lack of pooping.
Milk protein allergies in babies usually causes them to have blood in their stool or causes them to vomit a lot. An intolerance to lactose (which is in most milk-based formulas) can give the baby diarrhea, or bloating/gassiness. Soy formulas don't have milk proteins or lactose, so if you switched to these you should know if it makes a difference within a week.
However, my money is still on reflux. Do you ever here him gulping, gagging or choking after he feeds and then crying right afterward? Does he arch his back and scream? Does he spit up a lot (and, more importantly, when he does spit up does he sometimes cry)? Answer yes to any of these and it makes the reflux even more likely.
If you get to the end of this and you do think its reflux, then I think the best treatment is Zantac. You can try:
- thickening his bottles with some rice cereal (although if he is constipated, the rice cereal isn't going to help things)
- keeping him upright for a while after he feeds
- putting him on a baby sleep wedge or putting something under his mattress or the foot of the crib to make the mattress be on a slight incline (not enough so he slides down, but enough so his head is somewhat higher than his feet)
But if these don't work, then go back to the pediatrician and tell him/her that you are worried that your baby has reflux and it is making him miserable. Zantac has been used for a LONG time in babies and is quite safe. It's even used in many tiny premature babies in the intensive care unit. Depending on what dose your doctor starts, you will know within a couple weeks if it works (it takes a week for you to see the total benefit, and then if they started the dose lower they may need to try to increase it if its not working). And if it works, and it takes away your baby's pain, it's amazing.
Good luck! Hope he feels better soon!