He’s in great weight. He doesn’t need more. Would 50lb make him fat? No. But his BCS looks lovely.
I don’t even really see him lacking muscle. Many TBs will never be beefy. I’d like to see some more muscling along the top of his neck, but that takes years to develop, and he may simply be of a type that isn’t going to put it on there.
I would guess that compressed alfalfa flake is more than 3-4lb. I’d guess at least 5, and possibly 6-7. Assuming he’s eating a good 20-22lb of forage (dry weight), then 5lb alfalfa on top of that would be 25% of that, and 6-7lb is still in the ideal 20-30% range. Too much alfalfa can get some horses into trouble with some extra heat generation (not where we want to go this time of year), or some electrolyte imbalance (high calcium, though generally only a problem if that grass/grass hay happens to be high in ca, which is not common), and the calories can be excess energy instead of weight for some horses as well. Either way, this horse doesn’t need more of either, from the look and sound of it 
You COULD replace some of the grass hay in his net with some alfalfa, but again, that’s an increase in calories.
TC Sr would be a great option to try, with the same 10% fat, but a bump in protein. Or, you could add 1/2-1lb of a ration balancer, either the CarbCare Balance or, preferably, the ProElite Diet Balancer if you don’t have TC at your Southern States. TC30 is another option.
His issue may simply be that he needs more amino acids. Not necessarily more crude protein, just more lysine and methionine. The cheap easy way to do that is Tri-Amino or Nutramino.
