I prefer alfalfa pellets for added calories and/or rice bran pellets.
5lbs of Sr feed isn't enough. 5lbs of TC Sr is the bare minimum per day. It says to start out at 6lbs per day and increase as necessary. I would feed TC Sr and up it slowly to about 7-8lbs. It is a low NSC feed and can be the sole feed for a horse who cannot chew hay. In this case, it states to start your horse on 12lbs per day and adjust up as necessary, so it is safe to feed it in a larger amount then 5lbs daily.
If the upping the Sr feed doesn't keep him at a weight you're happy with, then add rice bran pellets or 1-2lbs of alfalfa pellets daily. But, I bet just upping the Sr feed will do the trick.
I wanted to add... the rice bran can also be fed in oil form.
The oil I prefer if I must use it is Cocasoya oil by Uckele. It is by far one of the healthiest oils you can feed. You can order it direct from Uckele or at a little better price at www.bigdweb.com. It smells DELISH and I've never had even the pickiest of horses refuse to eat it.