Is it normal for your 1yr olds testicles to keep popping inside him?

Actually, yes, it is normal for boys up to a year old to have testes that descend and ascend from day to day. However, if it's still going on at 18 months old, take him to a pediatrician or a urologist, to make sure that as the his genitalia develops and "seals", his testicle is on the outside.