I've noticed my aunts horse is sinking in a lot more. He usually is the type of horse to sink in a bit but I've noticed just today that he is sinking in a LOT more than usual. Now, he is having an issue with his left hind leg which could be causing it but I don't see how. My aunt seems to think he has an abcess in his left rear leg...do you think this could be causing him to sink in more than usual? If not what kinds of supplements can we feed him to "round" him out more? Thank you so much!

Assuming the area your talking about is between the last rib & beginning of hip.
First have his teeth checked [floated if needed] and do a fecal sample to see what type of worms he may have [to determine which chemical of wormer to use]

Feed-no supplement is going to be a secret formula. Instead make sure he has good quality hay available 24/7.
There are many different feeding programs but addressing the above is your best start.
Then depending on other variables you can adjust his current feed schedule.

I like Purina Amplify supplement and smartpak's weight builder, plus alot of great forage to keep weight on. It has worked very well for me and my OTTB's.