Help my toddler gain weight 😭

My 15 month old only weighs 18lbs. He’s not a picky eater by any means but he just doesn’t have the biggest appetite.

I still breastfeed, he eats whatever we eat and more (but in small quantities). We might not be the healthiest family but it’s not all junk food and he gets veggies and fruits daily.

Is there any smoothies or shakes or anything that he can have in between meals that can help him gain weight in a healthy way?