Hmm, I'm not really sure if I can answer this in a logical way. I think it's kind of a training issue as the colt I bought started out that way as did my husband's latest filly.
I actually really enjoy this personality trait because as you start asking them for more they don't have as much a tendency to get hot about things. My colt, as he got into better shape fitness wise has a ton of stamina, and My husband's filly has gotten more forward since he asks lightly and releases as soon as he gets response. Also as her fitness level increases she has much better stamina. She did take a few waps as a follow up so he could keep his legs light as did my colt and the lack of response ended there.
So I guess that it is depending on your complaint. If the colt isn't responsive then ask light with a back up if needed. Be prepared for a reaction, but that usually fixes the lack of response.
If the problem is a stamina issue it will get better as the colt gets into shape. I wouldn't let it worry me.
Feed wise I give my colt a 1/2 gallon of grain and extra hay as he is saddled, but that's only because I always give a bit of grain unless a horse is particularly hot and I think he could use the extra hay as he's growing and turned out with big horses.