Hi, I'm with NFU and although they are a little more expensive they are very good. I think it's worth the piece of mind in case you ever do need to make a claim.
I am with Scottish Equestrian Insurance and have been for many years and they have been fine. I have also had a policy with Petplan and they paid out no problem but they were a lot more expensive.
It might be easier to stick with the company he is currently insured with - especially if he is veteran. If it helps we're with KBIS - largely because we get as discount as BS members. We were SEIB before. We've never had to make a claim so I can't comment on customer service etc.