If your son is 13, in a growth spurt, and has outgrown his current bike, then he'll outgrow the next one pretty soon, too - a well-fitting bike is important. No need to spend top dollar for something he'll use for 2 years max.
Get an affordable aluminum race bike from whatever bike brand is stocked by the shop that you like the best. Dolan TC1, Specialized Langster Pro, Bianchi Super Pista, Felt TK2, etc etc - there are so many options out there.
Or just get a larger Planet X if your son liked it.
Or ask around at the tracks your son trains and races at - many race communities have a facebook group or a message board with stuff for sale.