What steps can I take with my FIL who is acting irrational and getting very angry at small things?

My husband and I showed up at his parents house to return a tool. When my husband asked where in the already opened garage he could put the tool, my FIL closed the garage door. Then ensued a big argument over whether or not my husband had told my FIL to open the garage or not (he hadn't, but my FIL will not/cannot? listen to reason here). Meanwhile my MIL is putting away dinner. She accidentally put a soda bottle in the top shelf of the door of the fridge. This meant that the door did not close and was slightly ajar. My FIL lost it when he saw the door to the fridge open, he started slamming the door to the fridge and wasn't satisfied until parts of the fridge started breaking. He doesn't hear well and he doesn't DO anything other than an occasional gig - he is a musician. Most days are spent in 'his' chair watching television. He is 71. We're not caregivers yet, but the signs are there that we may be caregivers soon. My FIL doesn't want to go to the dr. and I suspect he is verbally abusive to my MIL as she is very reluctant to ask him to do anything for fear that he'll retaliate. My husband and I are only children, my mother is 64 going on 45 and my dad is out of the picture, and we have no one we can turn to with experience. Can anyone suggest next steps for our family?