All horses are “sided” and go better to one side than the other. Is going to the left his stiffer side? This sounds pretty normal.
I’d say (without knowing more) that your horse isn’t really bent to the left and isn’t listening to your left leg asking for bend. Yes, you can use your thigh and inside left leg to reinforce the aid, but really be aware if he’s bulging that ribcage against your leg. Sometimes, you have to really take the inside rein and bend the horse to the left (immediately releasing when he “gives” to your aids) to remind him that can can actually bend. Also, you’re reminding him of where he needs to put his body.
I like to release the inside rein and expect my horse to keep the bend. If the horse doesn’t, it’s not bending/on the outside rein. Most horses start by not being able to hold the bend when the inside rein is released when they are learning the aids/aren’t very strong. But it’s a great tale-tell sign that your horse isn’t “there” yet. To help, I’ll turn in on the longside and legyield out at walk, trot and canter (horse has to listen to that inside leg and take the outside rein to be straight - no trailing haunches!). Or, I’ll ride a large square, using my aids to be sure he bends around the corners, not moves like a bus! That’s inside leg at the girth, outside leg back to control the hind quarters. No falling in on that inside shoulder!
Tipping the head out tells me he’s not bending, and much of the time that’s normal if the horse isn’t well developed and is stronger to one side. Show him the way to bend - it’s not rocket science and isn’t difficult for him. Don’t let him fall in on the inside shoulder to avoid working. If you are concerned, a chiropractor can tell you if he has an issue related to flexing to the left.
Stifle weakness can be helped tremendously by poles on the ground and GRADUAL grades (not hills to start).