L.L.
You know, symptomatically, it sounds just like my child's symptoms when she is eating gluten. She has Celiac disease. A severe gluten intolerance can cause the same types of symptoms...if I were you, I would consider putting him on a GF diet immediately just to see if it helps.
I'm telling you, when my daughter was 10 months old she hadn't grown in over three months (not a cm, not a gram), had the worst dermatitis I have ever personally seen (some people might mistake it for eczema, but it's much worse and painful), had TERRIBLE tantrums ALL THE TIME...for hours on end, with thrashing and screaming (well, duh, she was in pain. I felt so awful!!!), and almost constant greasy, oily diarrhea.
Within a week of having her on a strict gluten free diet, her symptoms pretty much disappeared. I went from having what I thought was just the most difficult baby ever to having a normal, sweet child...it was like a miracle to me. I cried and cried for the time we'd lost and for the joy of discovery. Of course, I had to put her back on all the food for testing and that was another torture in itself...and it proved it to me right there, because the symptoms immediately returned.
You CAN do an easy gluten free diet for a week, just to see if it helps. If you would like help planning for a week and a beginner's guide on what to eat and what not to, please PM me and I would be glad to help. We've been (my youngest and I) GF for about a year and a half, now, and it's made so much of a difference for both of us.
Oh...and DOOO keep pushing the issue. I had so many people tell me, "Oh, it's a phase" and "Oh, she'll outgrow it." No, she won't. Ever. People had me feeling the same way...please show this paragraph to your husband. A mother KNOWS when something is wrong, and yes, something is wrong. Oh, and if the gf diet doesn't work, you are right to check into other food sensitivities.