Question about a thoroughbred?

I have a old thoroughbred gelding the vet said he was 19 if not older it was to hard to tell. I rescued him a while back he weighed about 400 or 500 lbs and was about dead he couldnt stand up without fallin back down he was abused and straved since ive owned him he has gain about 300lbs he is startin to look good but he has stopped gaining weight he gets 6 qts of gain a day plus all the hay he wants ive started givin him a half of cup of corn oil twice a day in his food and it seems to help alil but is there anythin else i can give him to gain his weight back he has been pastured wit my 8 yr old proud geldin and my 2 yr stud colt but they let him eat hay anytime he wants and i give him his gain in his stall since he gets alot more then the other 2 but how long will it take him to get all his weight back and look nice in fat please no rude answers ive owned and rescued horses for yrs i just havnt had a horse take so long to gain his weight back like flame is doing and i kno tb are hard to keep weight on