I was just in that boat. I bought a 20 year old Yamaha PW50 off a buddy and rebuilt it for my son’s 7th birthday. He is about the same height as your grandson but half the weight. The PW50 is slightly small for him which I wanted and I plan on using it for his younger sister and brother in the future.
I looked into buying a new PW50 since the resale is so high but out the door price is $2600 around here, $1000+ in fees on a $1600 bike !
I think your grandson is probably too big for a PW50 or a JR50 and should at least go up to a 4 stroke 50 like a CRF50, TTR50, etc. or possibly a small 80cc bike.
Here is my sons bike.
