i would double check for a bean or any sign of infection, just to be sure. peeing 5 tmes in 40 mins is definatly not normal. there could be alot of causes. frequent peeing can be a sign of a bean, a urinary tract infection, bladder stones or in some cases kidney problems. the horse will pee little and often because of the pain. it may seen like he is peeing a large amout, but bare in mind a horse can produce between 10 and 25 litres of urine a day, usually urinating 4 to 6 times a day.
is he uncomfortable when you touch 'down there'? is there any heat? if you cant find anything obvious i would get the vet out. he may want to do a urinalyses to check for infection. hope this helps..