I am assuming you have actually entered his number in the BSJA horse search bit? If so and it still doesn't come up and you don't know his registered name or that two doesn't bring up anything then i would drop the membership office an email. My horse has been BSJA registered on and off (on now) since he was 4 yet, if I try and search by his name it does not bring anything up. Putting his number in however does.
I emailed them as could not find his details and they sent back an email telling me all his info inc notional and actual winnings etc (I think you're actually supposed to pay for them!) so it was very helpful.
So, I would try this, i just don't think their online search system is very good or whoever's input some of the info has done it in a way that you can't find it!!! My horse's name is STILL not coming up even thoigh he was re-registered last year!