Need help for my son's TTR90 suspension

I trailride with my son (10 years old, 70 lbs). He rides a '00 Yamaha TTR90. Needless to say, the rear suspension leaves a lot to be desired. When I follow him, the rear wheel is spending as much time off the ground as it is on the ground. The spring is stiff and damping is terrible. What I am looking for is any thoughts on aftermarket or other OEM shocks that might fit the TTR90 that would make the ride more plush.

Thnaks,
Eric K
'01 GasGas 300XC
'00 TTR 90 (son's)