This one is a bit of a pickle, because the method you’re using to teach him is the same one I’d recommend- break the problem down into its component parts, then solve each bit one at a time.
His issue probably isn’t total inability to break things down into parts- He can probably tell you that his arms and legs and head are parts of him, but not all of him- but rather just how abstract mathematics is.
I’d advocate maybe trying to find some way to make the examples more concrete, depending on what kind of maths he’s doing at his age. As a basic example, the ‘number line’ method for subtraction- take the lower number, write it on the left side of a line, take the higher number, write it at the right. Then count up from the lower to the higher to solve the question. You could teach him a different method for each type of math problem. While I, personally, do not like this method, because it makes everything very disjointed, it COULD, THEORETICALLY help him distinguish between the different components of larger math problems, and make it easier to break them down.
Another method you could use to split things up would be to have him write different symbols in different colours- as in, all plus signs in black, subtractions in red, etc. It could reinforce for him that these are DISTINCT parts that he needs to solve DIFFERENTLY. Similar to this method, you could also try to teach him to LITERALLY break the problem up into pieces. So, to use an awful example, 2*20-((3*4)+7), teach him to literally write down the different parts of the questions on different parts of the page, so he gets 2*20-( ) (( )+7) (3*4), and can just solve one chunk at a time. Dependent on his specific difficulties, this may be too much of a stretch, but I used to use the same method at school. It drove me mental when I got older and had to start doing my calculations ‘neatly’. I just didn’t think in neat straight lines, I thought all over the place 😛
Finally, you are right in that medication, additional teaching at school, and so on will probably help the young lad, but at the end of the day, more teaching can’t hurt, as long as you teach him the right stuff 😛
Hope this helps 🙂