I didnt read any other posts but here would be my suggestion:
Keep him on the senior grain, but not too much. Do you have him on a weight supplement. I would suggest putting him on weight builder, or a supplement like that, it really helped my friends skinny horse. Also he need to be getting alot of hay. Hopefully he will start gaining weight! I think that the weight supplement will really help, hopefully. and the beep pulp is good also :) I he eating in a stall. If other horses are eating with him that could also be a problem because he could not be getting enough of his food. Hope this helps.