The big singles can be pretty big, might be overwhelming for him to transition from a cot to a big bed rather than a toddler bed which is low to the ground then to a big bed? We got a second cot for dd2 with the same age gap, we then got a toddler bed about 6 months later on sale. We have now got her a big bed as were short a mattress and she was close to outgrowing her toddler bed.