Next bike up

Depends on how tall you are. If you're still small get an 85. If you're tall, or are starting a bit of a growth spurt, a 125 or 250F

Yep. What he said.
Measure the boy's height, and weight, and you'll get more informed answers.

My first guess would be the 85.

KLX 140, my wife has one, it is one of the best trail toads out there imo.
You said bike, so I am assuming you meant actually bike, as in size.

My daughter just went from a KLX110L to a KLX140L.
My wife and father-in-law also ride KLX140L's. They are terrific trail bikes that feel more like a full size bike than the TTR and DRZ 125's.
The next step for my daughter will be a KX100 as soon as she gets used to the 140.

My son went from the 110 to the KX65 then the next year I got him an 85SX and he's doing great. He was only on the 65 for one year but it worked great for getting him used to the power and clutch I feel

Jumping from a klx110l to a 250f sounds like a pretty fast way to the hospital. I went from a 85 to a 250f and still had trouble getting the hang of it.

I went from a xr80 to a rm85 and it was a pretty good transition.

pw80->65->85->125->250f->450f

It's time for you to go find a Kx 100

If it's really a bike for him to just ride around on, and there is no particular desire to go motocross racing in the next year or two, he should actually get the next bigger sized playbike up from a KLX110. That would be the TTR125/CRF100F sized bike. These have the same size wheels as an 85 but have the same kind of mellow powerplant he's used to. Both have a clutch that's a real clutch, as opposed to the optional clutch of the KLX110L.

If the goal is to end up on an 85 race bike, you could use the 110L as an interim bike for a short time because most kids seem to prefer a KLX110 as their pit/play bike along with their 85 race bike.

How good a rider is he or she, and are you planning to race? A small wheel 150r is a pretty nice bike for a mid skills rider, or an 85 if your kid can wring it out and ride it well.