angry looking horse at work

I work on a showjumping yard where one of the horses is always very grumpy. Ears back and with an angry looking eye - much like the photo (from Google as I don’t want the owner seeing my post!) but not with the white showing.

He’s nippy especially when brushing around his belly or tacking up (suggests ulcers or discomfort of some sort but the owner/my boss is not open to discussing this) and so stubborn when I’m asking him to move around when mucking the stable. There has been times where he’s trapped me in the corner threatening to kick so now I have to take him out.

Occasionally if I’m taking him for grass or when his owner feeds him treats his whole expression changes to ‘happy’. The ears come forward and he loses the hard expression.

I’m wondering if there’s anything I can do as a groom to make him a bit happier and nicer to be around? The people I’m working are at the top level and have been doing this since before I was born so I’m a bit stuck when it comes to doing any investigation into the deeper issue.

But would love for him to be a bit jolly! Especially as he’s so talented and not that old