Your baby's milestones are good! According to the milestones his age lies some where in between 12 to 18 months. Just observe him and give him time. Uptill now everything seems to be fine, just visit your Pediatrician after every six months till the age of 24 months, and get him vaccinated properly. Current weight is fine too as long as the baby is active.
From your answer I think there is no gross motor delay.