Doodles on the HVAC system

I love my son very much, but the way he finds a way to entertain himself is very concerning.

I think he is picking his antics up on our cat, who also loves to get in trouble, and I think they are now teaming up.

The cat destroys the toilet paper and my son follows suit. Recently, drawing has become his new thing. He started drawing on paper which was very nice, but I guess he got bored with paper and then moved to start doodling on the walls. I scolded him and told him only to draw on the paper, and I even bought him his own drawing book. Unfortunately for me, he seemed less than interested in it. He tried for a little while drawing on the walls behind my back, but his drawings were very obvious. He eventually stopped, or so I thought. Eventually when taking out the trash, I happened to glance over and notice my heating and cooling system had bright colors scribbled all over it. My furnace and a/c were jammed in the corner of my garage and mostly out of sight, so it can be hard to spot, but upon closer inspection, it had marker scribbles all over it. I instantly brought my son out and scolded him again, I sent him to his room and took away all of his markers. I then took a wet cloth and thankfully the scribbles came out nicely, and the HVAC unit looked almost brand new. I kept my garage locked for a while in case my son tried to go back and do who knows what to the HVAC system, but thankfully he seems to have forgotten about it.