Make sure he gets enough roughage such as good quality hay. What feed are you thinking of putting him on? If he is super skinny and you doubt he's been on any feed I would suggest starting him back on it really slowly. I wouldnt suggest oats or sweetfeed as they really don't offer a lot to horses weight gain wise, try a complete feed like strategy or ultium or another complete feed to put weight on him. If he is still thin once he's been getting fed proper for awhile you can try a fat supplement too but start that up slowly as well as horses can be really sensitive to feed changes. Good luck with him, I hope you get him healthy :)
Strategy is not a complete feed (senior and tuff like that is a complete feed, when it includes there daily forage needs if feed correctly)
I took in a underweight thoroughbred the end of december. He's already looking much much better.
Here is what I feed and it works wonders (you horse may require different amounts and should be started off slowly to reduce the amount of digestive disterbance)
He gets daily (split between two feedings)
All you can eat local grass hay
14 lbs alfalfa
3.75 lbd purina strategy healthy edge
3 lbs beet pulp (soaked and only given with grain ration at night)
2 scoops cool calories 100
Like I said this may not be what your horse needs or he may require less. The best thing is to remember once you have him eatting well and on a diet you like feed him the amounts for his ultimum weight not the weight he currently weighs in at. Good luck!
I have followed a very similar feeding routine as TBLover. But instead of cool calories I used Seminoles Ultrabloom stabilized rice bran.
We feed coastal grass hay and I also supplement with 4 lbs(dry weight) of alfalfa cubes fed once daily soaked.
It put the weight on at a moderate and steady rate over about 5 weeks I have noticed a wonderful improvement. I will be slowly reducing the ultrabloom to see if he will hold the weight with his other feed and I also started him on Smart Omega Ultra.
The ultrabloom is pelleted and I have to say it's a great deal easier to feed then loose. But I do mix it in with Seminole Compete safe which is a textured/pelleted feed with herbs and he eats it right up.