Baby sweats

Veronica • George Alek Jr 4/08/17💙 Layla Jae 8/16/2018💜 Currently pregnant due in November
In his car seat even if he's just in a onesie. We take him out and his whole back and head is wet. I know it's not recommended to put anything with car seat that it didn't come with but idk what to do except get a new liner for him to lay on.. any ideas on what I should do?