Did you breed him or buy in? If you bought him then what you paid for him or if you bred him, I usually cover stud and vet charges so insure all mine for £1000 from foals and up their value as they get older.
Remember no matter how much you insure for they will only pay out on market value so no need to pay too much plus the premiums take a hike after £2000 and you might need a 5 stage vetting if more than £5000.
Don't forget it's cheaper to join the BHS as a Gold member which gives you free liability insurance than it is to use a horse firm plus one membership covers all your horses not just the one as long as they're not used in a professional capacity.
I bought him for £600 but was a rescue, but apparently he is worth about £2500 i was thinking £1500 was a fair price, i dont see the need of him being insured for that price or is there?