excessive urination

my yellow lab/shepherd mix is about 5 or 6 months old and he urinates almost every hour. he tries to hold it but sometimes he cant. i do not think it has anything to do with marking/spraying or anything of that nature because when he "goes", he lets a lot out. does my dog have a small bladder? is this normal for a puppy to pee a lot? let me know what you think. thanks!