If you want to grow out the fringe you could try parting his hair on the side and sweeping the fringe across and behind his ears as it grows longer. Often with young kids' hair it can be trained to go a certain way. Or use an alcohol free gel. Maybe get him a baseball cap for wearing back to front around the house.
If it were me I'd be growing out the fringe. IMHO fringes on boys can easily move into mullet territory which is so not a good look!