I have a 12 year old who is growing like weeds. He is about 5' 5" - 5' 6" (1.65m) and has grown over 5 inches in the last year or so.

He has a cheap Fuji steel mountain bike (17" frame) that's heavy as a tank and we go for rides around our little downtown to the library, starbucks and such or longer rides on MUPs and some light trails. Our longer ride together was just shy of 30 miles with several above 20 miles. We usually average about 10 to 12 mph and I've put more road worthy tires which have improved our speed on paved surfaces.

He wants to do some longer rides and maybe even try a century with me this year. I have no doubt that he is capable as he is a good athlete (soccer). The only thing holding him back is a decent road bike.

I've been reluctant to buy him a road bike because of his rapid growth and I think that he would outgrown whatever I buy him now before the end of this year. A friend suggested that I buy him a bigger frame and try to fit him the best as I can until he grows into the frame. Replacing parts such as stems and raising the seat.

For reference I have a Fuji Gran fondo size 55cm (LBS guy says it compares to most manufacturers size 56cm) and he can stand over it with some clearance over his jewels. If I lower the seat he can pedal with some ease but he is very stretched out while reaching the bars which are above the seat level.

LBS tells me that right now he would be best served by a size 50 or 52 depending on the model. I was thinking of buying at least a size 54 so he can have use of it for longer. Should I go this route? If so what should I replace other than a shorter stem?

I've been looking on craigslist for something his size but no luck so far.