I started on a Z50 at age 5 for a year or two. At about age 11 dad got me a 1980 CR80 (this was in 1983). I am 5'4", 140 lbs now - I suspect I was about your son's size at age 10 or 11.
I wouldn't worry so much about seat height in inches - just take him to the shops and see what fits. You want him to be comfortable, but as you mentioned, give him some room to grow.
I would think that being able to touch the ground with both tiptoes or a little more ought to be sufficient.
Neither of those bikes will touch a 2 stroke 85 on the track - I'd guess the TTR90 is about 8-10 HP and a stock 85 is at least in the high teens in HP. An even bigger problem is that there's no way the TTR type bikes are going to jump the same jumps that the 85s do - the suspension just isn't there for it.
However, my local tracks have a trail bike class - 100cc and under, no 2 strokes. The Track Commissioner's younger son moved from an XR100 to a CR85 late last season and actually got slower because he was afraid of the bike at first, and then he couldn't seem to figure out how to "keep it on the pipe".
Given your description of the situation I would recommend buying a used trail bike for now - the XR/CRF/TTR type bikes are almost indestructible and generally don't require much maintenance other than oil changes and chain adjustments.
Then in a year or two you could get the MX bike. I suspect it will be a year rather than two. . .
After the older CR80, dad got me an XR100. It took me 6 months at age 13 to be totally bored. Then he got me a brand new 1986 CR80. It kept my interest for 3+ years.
He can learn to use the clutch - it's not that hard for a kid - a couple days and I'd say he'll have the general idea - I hadn't used a clutch on a bike and moved from the Z50 to the CR80, so . . .
Hope this helps . . .