My dog urinates normally most of the time but occasionally he needs out every five minutes and only does a series of squirts. He is a two year old labrador who has been castrated but the problem has been present since before his castration. Any ideas

A couple of things come to mind:

Scent marking? Even neutered males still like to 'paint the corners' of the yard. My neutered male still marks after both females and any area that is new to him.

See your vet immediately if there blood in the urine or if he seems painful when urinating. Could be something like bladder stones, bladder infection or worse.

Is the stream steady or pulsating?

Even if somebody here managed to guess the problem, likely the cure would still need to come from a vet.