Bucko...Poop!!

I really am starting to worry about my little man, he seems to get some bad constipation at least once a week, so bad indeed that he pushes randomly throughout the day until his face is bright red and his body is stiff as a board to gain nothing from the effort. He goes meal after meal not eating but maybe a bite here or there, so i imagine his tummy hurts. He screams out in pain, and reaches for me to hold him and rub his back to offer him some sort of support system during these times. I try everything, i change his diet, fruit usually does the trick by the end of the day, and i heard plums may also get the process to go along. He gets plenty of fiber, plenty of fluids, and i just worry that its something deeper that i cant see physically. It doesn't help when we go away or leave him over night because then he forceably holds it in until he is home, and well it makes it worse. Today he worked all day trying, nothing. That is until bath time, it was although the hot water sped things up (i am actually glad for his sake!). Although it was our first poop incident in the tub, i wasn't upset at all, i was prepared just couldn't reach him in time to plop him on the potty. He sat on the potty afterwards, with his little towel wrapped around him and this sad embarrassed look on his face, i smiled and reassured him that it happens, and i am glad he is feeling better! If anybody else has an idea on what can speed this process up that would be lovely...i'm willing to try most anything to help him feel at ease.