I have a little quarter horse that I rescued. He's filled out now but his starvation during his first 2 years stunted his growth and he looks like a pony next to my other tanks. My saddle kept rolling on him too while mounting. Thankfully, I tend to be a hoarder when it comes to tack so I was able to drag out my old youth sized saddle to use on him and voila problem solved.
Your saddle will probably be fine on your horse once he gains some weight & muscle so in the meantime maybe ask around to your friends to see if they have a youth sized saddle gathering dust in their tackroom that you borrow for a few months.