You won't get a float bike in his size but you can still take the pedals off the bike he has. Once he can balance and steer without pedals put them back on. You want long stretches to practice on, ideally gently sloped grass then faster downhill surfaces. If he is embassased to remove the pedals then leave them on and get him to just not use them for awhile.
My DD was nearly 5 when she got her float bike, she rode it sans pedals for at least 9 months (anxiety at play) and then went straight to riding it just fine. She had a few dramas learning to correctly position the pedals before she tried to get started but could balance, pedal and steer from the get go. She needed a new bike within a week or two of using pedals as the float bike had short cranks and no gears and she could not go fast enough to keep up on family rides, which she was instantly ready for.
