Your son shouldn't have kicked, but he's 2 and you are teaching him to not to do it. You've managed to produce a well behaved 5yo so your methods work.
Your FIL kicked him back, which was common 20 - 30 years ago. If you take the view that that was his way of handling it and that's where he was coming from, even so the proportionality of his kick (leaving a bruise) was too great.
Your FIL is absolutely not listening to you though when you say you don't want your son disciplined in this way. These are your sons, and your parental method of disciplining should be respected generally, but specifically in such a controversial area as physical punishment.
"he has a lot on his plate" is an unacceptable excuse for leaving a bruise on a very little boy's shin. It is an excuse too; the people saying that know it shouldn't have happened.
He also spoke to you in a really unpleasant manner on the doorstep.
The crux of the matter here is that FIL is unrepentant and not accepting how you two want your child to be disciplined.
From what you said, you have a FIL problem and he's not open to reason. As someone said, maybe you will get further if your DH speaks to him. But possibly not.
In that case, I'd be very wary. Firstly cut down the visits and secondly when you do visit keep your little one physically away from FIL, you and your DH. If FIL doesn't like it - well, when he respects your limits, then the rule can be relaxed. It's not going to be easy in a practical sense mind you; but the alternative is accepting that that is how FIL disciplines, and let him get on with it (which I suspect is not okay by you!)
Fwiw I have a FIL like this, old farming type. He uses a kick or a slap to discipline the dog, and did the same with his own kids. He knows that no way would I tolerate that and so he makes (for him) a big effort and does not do that with our children. I can see how much of an effort he's making since physical discipline is very much his natural experience, and I appreciate that effort. It also means we can go visit him more because I can trust he -won't- hit.