If I can recommend, I would get a 75 gallon tank rather than a 55 gallon tank because 55 gallon tanks tend to better for fish, not lizards, because of its narrowness. 75 gallon tanks are wider, apprx 18" wide, but are the same dimensions as far as height and length go for a 55 gallon tank. Lizards of that size need the space to turn around as adults.
Putting your baby in a 75/55 gallon tank now is quite alright =) He may go through a period of relocation stress, as mentioned above. The only other issue you may have may be feeding. When it comes to feeding time, just make sure you section off a portion of the tank or feed him outside of the tank in a smaller Tupperware container so that he may catch crickets and they don't go off and run under cage furniture and such before he can.
Otherwise, is it perfectly okay to move him into the larger tank.
-Glue