Re: what should i be feeding my 19 year old 15'2" heavyweight cob gelding?
I also have a heavy cob, a mare who is 22, who lives out 24/7.
By the sound of it, your boy is missing his turnout, especially if he has gone from full turnout to only 4 or 6 hours a day.
My old girl crib bit before we had her as she got bored and moody being in a stable, even if only over night, so we've always kept her out, and she's been absolutely fine. If ever she has had to be stabled, even for a couple of hours, she will kick the door, start weaving, and generally misbehave.
Rather than mess about with feed he probably doesn't need, I would ask if he can go back to 24/7 turnout, or if not then look for somewhere else where he can be out all the time - it's obviously what he needs, and the bad behaviour is his way of telling you he's not a happy chappie.
Good luck with him, and I hope you can sort it out.