Feed/energy/ hard to explain in this little space...?

so heres the story. ive had my horse for about a month & a half. as of yesterday he weighed 1050 and id like him to be 1100. right now hes on purina strategy (6 lb per day) & pasture 24/7. he gets worked almost every day, not hard, but still a work out. he gets worked hard about once a week (2 hour lesson). when i went to see him my first time (deciding to buy) he was much lazier than he is now. now hes full of energy & life, which is fantastic, but in your opinion, is it a good idea to stay with purina strategy or should i switch to something that will put the weight on faster. i know the whole pasture board thing probably added to the change in attitude (he came from being stalled with one other horse in the barn, now hes in pasture 24/7 with 6 others) but is it necessary for him to be getting all this grain energy, or should i change to just help him put on the weight