The is a big difference between the Mini Adventure and the Pro Senior. If you aren’t planning on him racing, I would go the Mini Adventure as they are air cooled and can be ridden all day. Where as the Pro Senior is a much more highly strung water cooled weapon. The Pro should only be ridden for short periods as they are designed to do a race and then go back to the pits to cool down. Continual riding tends to get them too hot and the stator will burn out. $160 thank you. I am talking from experience, as we have had two pro seniors and it happened to both.(Still have one here in the shed).
The Pro senior is height adjustable at the top of the rear shock but you will only get it about 1 inch lower in the rear and and inch lower in the front by dropping the forks in the triple clamps.
Another bike to look at is the KTM Senior adventure, it is also air cooled but a bit taller than the mini adventure, it has the same engine as the MA but the height of the Pro senior.
You can get some extra sting out of the MA and SA by fitting a pipe of a Pro Senior to them, they will go like a shot dog then!!!