Has he seen the vet yet? :) I think that it should be your number one priority for him.
Alongside the obvious questions on what feeding methods the vet recommends, I think there's some points that you need to talk with your vet about.
#1: TEETH. It's fairly safe to assume that Switch is in dire need of a teeth-floating! You might even want to schedule a his teeth to be floated on the same day of his examination, to just get it all taken care of.
#2: PARASITES. I'd also say that it's extremely likely that Switch needs to be dewormed. However, it's best to ask your vet what brand/chemicals are best to use for him. It may be as simple as Ivermectin, but consulting the vet first, since he is in such an unhealthy state, it your best bet.
#3: VACCINATIONS. Because of the appalling condition his former owner left him in, I think we can assume that he has not been vaccinated. However, fall is not a good time to administer spring vaccinations, but it still may be a good idea to talk with your vet about any possible fall vaccinations that he or she might recommend. (I don't know how vets are handling it, but a google search revealed that West Nile has indeed appeared in Florida, so a West Nile shot may or may not be in order for your boy.) Also, it will be important to get his spring vaccinations covered on time! :)
Also! A weight gain supplement is a good choice for him, but nothing will replace a good foundation diet! As long as he's getting 20 lbs of hay per day (so glad that he's eating hay now!!) that will be a big factor in his recovery. In addition, if I'm not mistaken, you are feeding beet pulp, which, if you're feeding the correct servings, will be beneficial, too. Though a senior feed may be slightly more of a help than beet pulp alone.
You may also want to consider adding in flax seed oil, or an Omega 3 supplement. That will help his immune system, skin and coat, and just his over all health! :) An Omega 3 source holds priority over a weight gain supplement, though once the Omega 3 is in place, a weight gain supplement is a big plus!
Check out this awesome article! It provides a pyramid, and is easy to understand and read! :) Building A Better Diet | SmartPak Equine Blog