Fluids and proper temps are always the first things in a rescue situation. Now that he's had both, he'll show you where he's at rehab-wise. If he's curious, tongue flicking, etc. that shows he is probably looking for food. Do you have pics? Though it sounds like he's rather skinny, I'd still offer him a small meal like a rat pup and see if he takes it. Honestly I'd offer it as soon as he seems rehydrated. Worst that happens is he doesn't take it so you give him 4-5 more days to settle in and offer again. Eating is always better than tube/force/assist feeding. Now that he's had a few calories from your feeding him, he can probably handle regular food though make sure it's on the small side. Small meals will digest more easily for him. After a few small meals, then bump him up slowly until he's at regular sized meals every 5-7 days.

You're doing great with him so just keep it up. As he gets stronger, keep re-evaluating him to see what he might need. Each one is different.