How old was your son? I caught mine the first time when he was 11. By that time I had seen him on several occations playing with his ding-a-ling in the bath. That particular morning I was serving breakfast when I noticed him googling down my front. I was still in my nighties and didn't wear any bra so he had a very nice view of my tits. Right after breakfast he went straight to his room. I knew what was to follow and wham !!! when I tiptoed past his door he was on his bed jacking off.
My son was 5 when I caught him the first time masturbating back of the house. I ignored but later also found his teddy with a hole between the legs. He must have seen me and his dad in action.