i guess it all depends on the temp in his room, and your house's heating, lauren. we use them with a onesie on underneath during autumn, and as it gets colder, will probably add either a long-sleeved onesie and socks, or if the heating breaks down like last year, a full set of cotton jammies underneath! we also have a space heater, which can make it really toasty in there, so are trying to find the balance. i remember our hv said not to go by the coolness of their hands and feet, but the tips of their ears, if that's any help!