The only thing i think he could mean by that is to keep your lead foot pointing straight at your opponent, maybe when you jab and throw a right hand your over dedicating a bit and maybe that is what hes refering to with the straight leading leg and leaning comment you say he made. makes me think maybe this is the problem because over dedication and leaning seem to come hand in hand when technique isnt quite right. maybe your lead foot is pointing past your opponent on jabs and straight rights, etc and thats what hes picked up on?

If he is literally telling you to keep your lead leg straight and locked then i would maybe find another trainer because that doesnt sound right to me either, you probably know more than us if your trainer is any good but either way if your second guessing your trainer already then thats not really a good sign either, ask him to explain why the leg needs to be straight and what he means by it. if he has an explanation then please share if not, new trainer lol. good luck.