I had to rehab a very malnourished horse back to health last summer and my advice is to cut out all riding and roundpen work at this point. It looks like Rowdy is starting about where my guy did. I made the mistake of jumping the gun into training (even just the ground work) too early and it really slowed his weight gain for the first few months. The pasture he is in in the pictures looks perfect. I would suggest free feeding medium quality grass hay with 24/7 turnout (or as close as you can get) so he can exercise himself as he sees fit. Then of course there are all the options for ration balancers (which I highly recommend), grains, supplements, and so on and so forth, but given your nutrition class I'm sure you're covered there. I would not recommend any training at all, beyond what is necessary for your safety such as basic leading, food aggressiveness, etc, until you are barely able to see his ribs any more. It will seem like a really long time, but, in the end, it will actually save you time because you will be able to progress faster in your training. Faster is not always faster in these situations.
Rowdy is a lucky boy. It looks like he found a soft landing. I can't wait to see him with some more meat on his bones.