18-month old hasn’t grown in months!

My 18-month old is very small for his age. While his head is almost off the charts at 98th percentile, his weight is 30th percentile and height is only 3rd percentile! He hasn’t put on weight or height in 3-4 months at least! Cognitively he is very advanced. Says over 100 words. Physical development was much slower however. Should I be worried he isn’t growing? His eating seems OK to me although he doesn’t like cow’s milk.