Help! Switched from infant to soy

Ok, I have a 5m old and he's always been on infant enfamil. I had problems with severe constipation on gentlease so my Dr had him on lactose enzyme drops (colief). He always had diarrhea, only one diarrhea poop a day. Severe gas too, woke himself up all the time. Finally I told my Dr I wanted to try soy and he asked, ...if it doesn't work (like he said) what you going to do then?.. BTW I switched him once from newborn to gentlease for 2m then to infant (with the drops) for 3m. He kinda pooped his first solid poop ever (about an inch) this morning. Now he hasn't pooped & is kinda crying weird. I'm so scared I went against my Dr (even tho everyone thinks he should b in soy) I'm not sure if it's OK. BTW again he was just sick and is still on ammoxicillin(?). What do I do or watch out for? Did I make the right decision? Help please..