I would double check for a bean or any sign of infection, just to be sure. Peeing 5 tmes in 40 mins is definitely 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 can't find anything obvious I would get the vet out. He may want to do a urinalyses to check for infection. Hope this helps..