We had a ‘tag a long’ trailgator style thing at first for my Nephew when he was 5.
Lots of patience and just running along side the bike when he was off the trailgator.
Practise is indeed what he needs and his riding will come in time. At the moment the weather doesn’t help, as children always prefer dryer, warmer conditions. Lots of praise when things went well and humour for when things didn’t.
We used to go to a picnic area, take food and drink, then we’d ride a short distance away from this area in case he got bored or had a hissy fit. My Husband and I would take it in turns to stay with the bags.
Eventually he was actually cycling along side us, then when things got hilly or too bendy, or he was too tired. the trailgator was at hand to hitch the bike back up to.
2 years on and he’s mtbing with us. So don’t give up!