Hi. My son has said "my brain made me do it" or "my brain told me to do it". It is a way of blaming something other than just saying "I did the wrong thing". What I'd say next time is "you arent' fooling me". And just move on. If you react too much, he'll keep saying it.
My oldest son has sensory integration disorder and he is prone to saying such things. He has had chronic low self esteem which as his mother kills me. I hate that and have tried to help. And frankly, he's doing a LOT better. But he'll slip in something like "I'm bad." "I'm stupid". And I'd react to it every time. Eventually a counselor told me to say "I love you too much to believe that." My son just looked at me and said "okay. That's true. I'm not stupid." But we had done a whole song and dance with this the two of us up until that point.
So my suggestion is to not do the song and dance.
Does your son have any issues you suspect such as adhd, add, or anything? just curious. Young kids that get in trouble often are more likely to say such things as my brain is evil or I'm evil or my brain told me to.
if your worried pray over him...it wont hurt...kids have a mind that sometimes things are hard to believe..but at times you cant dismiss it...pray with him and teach him to pray..see if he will feel any better..