Before anyone else replies I just want to say you're probably going to get ridiculed for having him in an adult enclosure of 18x18x36. HOWEVER I'm not against it. I have my 6 month old veiled in a 24x18x36 and personally I think it could be a "forever enclosure" no problem. I even know of breeders and cham keepers that only use 18x18x36 for all of their chams and they are all healthy and beautiful, so ya. At 2 months a 16x16x20 should be fine however he will outgrow it quickly. I started with an 18x18x24 and it lasted me until he was 4 months old, however I could have pushed it for another month or two with causing any cramping that would be detrimental to his health, but I chose not to. So that's you're call on how long you want to use the 16x16x20 for.