I am surprised you haven't had a lot of responses yet, but you will!
Guiness isn't testing you to see if you will leave him. But he is acting like a horse in a new herd; he is pushing against the herd members to see if they are above or below him. He has already estblished that your FIL is above him and apparently, that you are below him. Now bad feelings, that's just the way he sees it. And when your actions toward him are very mild, such as just leading him or asking him to walk, he tolerates them as acceptable from a subordinant. But when you put more pressure on him that would be acceptable from a subordinant, such as asking him to really move, he is insulted and tries to put you back in YOUR place.
That's where you need to wake him up. YOu'll have to surprise him and tell him "rule change!" (like Wife Swap, one of my favorite shows!)
You see, lunging or round penning isn't about making the horse go around and around for your amusement or for his exersize. It's about having a horse's mind in the correct orientation to the person who will then put their LIFE on his back. The horse you lead is the horse you ride. So , if he is running you over on the ground and not there mentally for you , he will not be there mentally for you when you are in the saddle.
When you next lunge him (and I assumed you were in a round pen, but am I wrong? Would be best if you could free lunge in round pen), you start him going around and if he comes in toward you, don't wait until he is close enough to cow kick you; go after him immediatley ! He starts to cut into the middle, putting his inside shoulder toward you, you leap up and down, stamp the ground AT him, flail the whip really madly and wake him up!
Of course, he'll buck and kick out and act really upset. Let him but don't stop until he leaps away from you. As big as he gets, you have to get bigger.
Once he has moved off of you, take a deep breathe (you will be shaking no doubt) and go right back to asking him to trot. Choose your direction and move him on. If he comes at you, go after him immediately. Don't let him start thinking that way. However, you don't go after him if he's just bucking out there , only if he comes toward you in any way.
As he starts to change his thinking about you, it will take less and less to remind him to stay out there. Maybe just a raised whip or hand and a strong "ah ah ah!". Move him in each direction, having him go up to trot, down to walk, up again in each direction. Canter him if he is ok, and when he lowers his head and licks and chews , stop and let him walk. If he walk toward you slowly and POLITELY, let him. IF he comes charging in toward you, move him back out. He must come toward you respectfully. And when he comes toward you, stop him before he comes closer than two arms lengths from you. Then YOU go toward him and pet him , put his halter on and put him away and leave him alone. Dont' cuddle very much, just a pet and good boy and walk away.