My son is almost five and weighs 39 lbs. Is this normal?

I'm asking because I only have visitation of my son. To be honest, my ex is a little lady who only weighs like 98 lbs. at 5'2" or so. He does kind of take after her in the eyes, pale skin, and seemingly weight department. Myself, I'm 6' and weigh 200 but at TWO I weighed more than my son. I've met several two year olds who weigh twice as much as him. I'm a bit concerned to be honest. He is pretty skinny. Not like ribs showing and starving skinny, but I've never seen a kid as tall as him and so light.

A little background: I know my ex eats like crap. She basically lives off of doughnuts and pepsi. My son tells me he usually eats Kix and eggs in a basket when with mommy. Additionally he's a pretty light eater. My family and I are always trying to get him to eat more but he only eats a few ounces of food before he gets bored and wants to go do something else. I'm concerned that he's used to eating very little junk food and that's it, but I also acknowledge he could just take after his mom and not have much of an appetite. I should say that my ex was accused of having anorexia a few times just because of her weight and appetite. Otherwise both her and my son seem healthy and energetic enough. Any advice?