Solids - In general the starting age has moved closer to 6 months for all babies because their digestive systems seem to handle it better. My kids all started earlier though. We started my daughter at 3.5 months hoping the rice would help keep things down. Unfortunately it only added constipation to the issues. My middle one started at 4 months and did fine. My littlest started at 5 months and was fine as well but we started VERY slowly.
Prevacid - This is a difficult question because it depends highly on the manufacturer and preparation. For example, the brand Prevacid tablets do not contain lactose; however, the solutabs contain "lactose monohydrate-microcrystalline cellulose sphere." But, many of us wind up with generic which means you would have to ask the pharmacist about the inactive ingredients each time you get the medicine filled.
Reglan - In general, my doctors have told me to do trials off the meds when you are no longer having symptoms despite gaining weight. It sounds like the subclinical amount is enough to be holding him off at this point. Did your pediatrician write the script for Reglan or was it a GI specialist? Were any other meds tried? In general, Reglan is now considered more last resort due to the side effect risks but of course, I don't know the entire story so in your case your doctor may have felt the benefits outweighed the risks.