It depends .... a child in diapers and then pullups is used to going anywhere, any time. If that child is just put in underwear with out having done the necessary potty training, they will continue doing what they have always done.
If the child has had potty training and has been using the potty successfully for a period of time....and then has started to pee everywhere then something is going on. It could be anxiety about something new, it could be a medical reason, etc.
so the main question is....had he been potty trained successfully and if so how long did it last?
I quite agree, with Sandman2 above, If the boy has not been 'Potty Trained' prior to being taken out of nappies, he knows no better. I used to be, quite friendly, with a couple who had two children the younger, a six year old boy, just used to 'Go', in his pants. He also wet the bed nearly, if not, every night. His Bowel control was, somewhat better, at least he know when he 'pood' himself.
Clearly, if the boy was clean/dry, for a time and then starts Wetting again there could be a problem- though he is quite young. I would maybe get him Medically checked but honestly wouldn't worry, too much, in such a young child. I have to tell you that, ALL THREE, of our children were 'in nappies' at this age.
Basically, unless this is a major regression, I would just put him back in nappies/pullups. Maybe Give It, six months, and start the Potty training again. If, in two years time, he still needs Nappies/Pull Ups, then DO ask for a Referral. For now just let him develop, at his own pace, accepting the occasional 'regression'.
Wishing you all well.