Hi Kelsie!
Personally, these are some of the most challenging patients for me. If someone has bothersome spasticity, I will do the standing frame occasionally to temporarily alleviate some of those symptoms and also help with autonomic functions. His 4/5 left triceps strength will be helpful, and you can work on press-ups and having him assist with transfers. Any kind of prop sitting and transitioning from anterior to/from posterior prop in short- and long-sitting would be good. You can do object manipulation while he is sitting to challenge trunk control (maybe propping on his RUE with full extension while placing objects in a basket with his LUE).
Again, it’s a challenging population and you’re doing all the right things!