Horse with a sore bicep

[Apollo1401]

Hi everyone, completely new to this forum and looking forward to getting some advice. My horse was recently diagnosed with Sacroiliac disease and has received the appropriate treament. He has been brought back to work slowly and had been going well until yesterday. He didn't appear to be his usual self under saddle and when I asked for canter he objected by trying to buck and running sideways (not his usual behaviour). When I trotted him up he didn't seem lame however when I pressed on his right hind (bicep region) he flinched in pain and kept moving away when pressure was applied. I have put him on box rest, gave bute and will not ride him for a few days.
Anyway, I'm wondering if this is the right thing to do as I'm wondering if it's a trauma related pain or if it's related to the recent sacroiliac disease diagnosis.
Any advice would be greatly appreciated.

 

[skint1]

I wonder if he might be better with turnout? Would he not get stiff and stock up in a box? My vets are always happy to take a call or text to answer questions so it might be good to get their input so you know the best way forward in any case