All of my children get consitpated on Rice Milk. In fact I always keep one on hand in case they have diarrhea because it binds them right up.
As for Benefiber, yes it truly is tasteless. You can add it to any liquid or pureed food. I put it in Myles oatmeal. I usually only give him 1/2 tsp once per day, but he’s quite a bit younger than Seth. I’d probably start with a lower dose first and see if it helps, then you can work your way up if it’s not enough.
The Lactaid trial sounds like a good idea. If he could handle that than maybe his consitpation issue will resolve on it’s own.
Good luck.