I'm not a vet (disclaimer) but I'd guess that a diet moderately low in starch/sugars would be OK for this horse. Giving a big bucket of grain to a starving horse is a recipe for problems, whether the horse is actually predisposed to laminitis or not.
My understanding is that high protein causing laminitis is a myth- things that are high in protein (i.e. alfalfa) are often also high in calories, and obesity is a major risk factor in laminitis, so feeding alfalfa to an overweight horse CAN cause laminitis, but it's because of the calories, not because of the protein. In general, alfalfa is lower in sugar than most grass hays, so it should be safe to feed to an underweight horse, even one who has had a bout of laminitis.
You'd probably also be fine feeding him a moderately low NSC hard feed (<20%). Triple Crown Senior was already suggested, and is a good one for both low NSC and putting on weight. You probably already know, but be sure to make feed changes very slowly- perhaps even more slowly than you normally would, since he's already in a bit of a delicate state right now.
If you find he's not eating enough to keep putting weight on, you could add a high fat supplement like flaxseed (40% fat), rice bran (20% fat), or flaxseed/canola oil (100% fat, but some horses don't like the texture)