Diet advice to help a sheep gain weight before the winter

I bought a ram and with him came some resistant worms (don't worry, if he's not better in time for breeding, I have a back up planned).  He had the shits for ages before we found a way to remove the parasites.  But he's better now.  Only super-skinny.  

He needs to gain weight and fast.  

What can I feed him?

With llamas and alpacas, I find beet pulp mash works wonders.  It also helps get more minerals into them.  Can sheep eat this?

Of course, I'll adjust their diet very slowly to prevent bloat.  

Grain. Introduced slowly.
Is better crushed.
We get the odd bag of cool feed/maintenance mix for our horses and goats. We just give enough to keep friendly or extra if someone needs it and it's the cheapest feed which is still very palatable.

I agree with Andy but I recommend reading this article, it covers both sexes nutritional needs as well as those of the lambs.  feed needs of Ewes, Lambs and Rams there is also a PDF you can print out for reference.

Redhawk

He's already on grain rations.  I'll be increasing it a bit more, but I can't do too much or it will cause more harm than help.

The rest of the flock is fine.  Exactly where they should be for this time of year.  It's just this one fella who had the parasite problem.  He's already on a variation of the diet described in the link - but with changes to account for local conditions.  The problem is, with his battle with medicine resistant parasites, he's going into winter with not enough flesh on him to survive.  The fact that the rest of my flock were healthy enough that they didn't pick these bugs up makes me very happy and confident that I'm on the right track with my nutritional programme.  

But how to go above and beyond normal diet to help this guy gain weight?  Looking at that link, his body score would be zero.  

I've separated him from the rest of the flock and gave him some wethers for company.  I tried them on some beet pulp yesterday.  Only one liked it.  Not the one I need.  I'll try again later.

I'll also try adding a higher quality hay to their regular mix.  Alphafa is my first choice, but he's a Cotswold and more sensitive to high nitrogen foods than my other sheep, so there could be problems there.

I'm wondering about rumen bacteria.  I'll see if the feed store has something like a probiotic for sheep.  If not, I'll have to do that thing where I take a cud out of a healthy sheep's mouth without losing too many fingers, then feed it to the poorly sheep.