Going on the premise that the horse really needs the extra weight (I know far too many people who think their horse is skinny when it's at a healthy weight). E.g. If 6 is perfect, 5 is better than 7, but 7 is better than 4, etc..., but we won't get into the health issues that come with weight for horses. I'll trust that you know and your horse could be at 5 wanting to be at 6 (we all love perfection).
First thing to remember is never change a horses feed suddenly. Transition to it at least over a few days.
More grazing you can do right off, since that's just more of what he's already getting and barring anything unforseen (e.g. Too much time on lush grass, etc...) it should be safe.
If you want something to transition to that will put weight on them the two most digestable feeds I know (not counting oils) are beet pulp (a good, but under used feed in my opinion) and copra (and they compliment each other in the Ca to P ratio). Beet pulp is an easily digested fiber that is taken care of in the hind gut. Copra is easily digestable and done in the foregut. Good for hard keepers, low in NSC, but you have to be careful with easy keepers unless you're working them enough (or feeding less).
But as already said by most of the posts here. More grazing is what you can do right now. Horses left to their own devices in a pasture will graze well over half the day (e.g. 14-17 hours) so 2 or 3 hours is relatively very little unless you have free choice hay in front them (and more hay might help put some more weight on too since that's not a lot of hay to go with a few hours grazing).
Of course in todays world of "what can we sell to the equine lovers out there that will buy anything labled 'for horses'
" there's no shortage of things you can buy. There are LOADs of suppliments and feeds out there. I promise there's somthing that will claim to do whatever you need. I've even saw a friend had a jar of tea tree oil repackaged as a cream "for horses" at higher price (of course) than I pay for the oil