Do you have the 50 on a speed limiter? My boy is almost 5 and I don't think hes big enough yet for the 90, but the 50 looks like it might be too slow/no fun.
Just get the 90 or a DRR 50. My son is 4 and the 50 is to slow for him already. This video is as fast as it goes the with the speed limiter it will only do like 7 mph.