Get him in a circle on a strong bend. When you feel him actually moving over ask him to canter. It is hard for the horse to pick up the wrong lead while bending hard.
The horse might be stiff on that side, or it could be a stifle problem. If not, it can help if you lift the inside rein a bit when you ask for the center.
Both legs give a little wake up squeeze to let the horse know he is about to be worked more, feel of the bit by a tightening of reins (only a little), inside legputs pressure on the side, and give a little kick with the outside leg.
Get his stifles and hocks checked, this issue is usually related to pain.
Ask on a tight circle and make sure you’re pushing your horse really forward. If you’re really struggling under saddle they are more likely to pick it up in the round yard and just practise it like that until they’ve built up strength to do it under saddle. Also make sure you’re not doing too much work in the other/stronger direction as that could also be making the other side weaker
ask for canter in a corner
try picking it up around a corner or in a circle
This might be horses stiffness on that specific side. You can try riding lateral work and riding a square to make horses shoulders more supple and encourage them so the horse can easily get the correct canter lead. It might take time but should help.
Try to bend and get your leg really behind the girth. You could also do flatwork rollbacks where you change direction and go into the csnte
Try with a lot transitions walk-trot, small and extended trot, then when you feel the impulsion try trot-galop transitions. It should work.