I know that sometimes for the girl that I help give lessons to she has a hard time seeing when the outside leg is up, so instead I tell her to be sitting when the outside leg is underneith her because that is easier to check for. You could try checking for that!
You will still be rising when he is reaching forward with his outside leg but you may be able to see his leg going back (thus when you sit) better then checking for the leg to be forward. Another thing I do for her when she is having a really hard time figuring it out is I stick christmas bows on her pony's shoulders, you should be able to see the bow move forward when the leg reaches out so you know when to rise and fall.
Once you know when to rise and fall you will eventually start to feel the diagonal. Your hips will swing backwards one at a time with the rhythm of the trot, much like if you were pedaling a bike backwards, every time a leg reaches out. One thing my old trainer did waaaay back when I was learning diagonals was to put me on the lunge line and just have me post. She would ask me what felt more comfortable, without telling me whether or not I was on the correct diagonal, and I usually picked the correct one. This is because it helps balance us and our horse. Just play around with different methods and see what works best for you!