Not quite sure what you're saying. Is what you've said what you currently feed & you're wanting to put the horse on pasture instead? & you're worried about him being/getting fat?
Without knowing how big your horse is, etc, who knows how much he may need, but if you've got a big/rich pasture to put him on, I'd definitely consider fencing to restrict grazing, such as strip grazing or if poss, a 'paddock paradise' type track. A grazing muzzle &/or only part time grazing may also be a consideration.
Re your current feed, again, who knows how much he needs because we know nothing about him, but horses should have free access, or at least little & often feeding of roughage/hay. Does he polish it all off? Does it take him most of the time between meals or is it gone quickly & he goes hungry for some time? Even fatties on diets need to have at least 1.5% bwt daily in roughage. While that may be enough overall, I'd consider putting it in a 'slow feeder' or small holed/doubled hay net and certainly give him more than that over night. If you're worried about his weight, soaking the hay for a few hours & draining it before feeding will leach out some of the excess sugar.
I personally would avoid feeds such as Mane Menu, who's top ingredients are wheat middlings(sweepings) & corn. I tend to stay clear of grain & other starchy, sugary feeds where possible, but if you are going to feed these ingredients, little & often is important, rather than feeding only 1-2 meals daily.