C.G.
Most people (around 70%) who are allergic to milk are also allergic to soy, so it is probably not an allergy. LActose intolerance has varying degrees of severity and is actually pretty rare in young babies- it is something that builds up/gets worse over time.
Soy formula causes constipation, BIG TIME. Soy formulas are meant to be a last resort because they are inferior to milk formulas in nutrition vs benefits. Probably he is just adjusting to the formula. Give it at least a couple of weeks.
Also, all formulas, generic and name brand are pretty much identical (by type, soy vs soy, etc). If a doctor recommends a particular brand it is generally just that the doctor has a relationship with the selling rep from that company and gets benefits from them. So I would take medical recommendations with that in mind- I work in the field and have had many a lunch paid for by formula reps.