I have a very busy, easily distracted 13 1/2 mo...

I have a very busy, easily distracted 13 1/2 month old. He has always been lean (now approx. 9.8kg) but is also tall (approx. 80cm) as dad is very tall. We battle for probably 6 out of 7 days to get him to eat a decent amount of food. He will not sit in the high chair for very long without losing interest and then crying. I try standing him at the coffee table but it only ever lasts for a few minutes. I have offered everything imaginable to take his interest but he will only eat tiny amounts of anything. I give him plenty of finger food and he has his own spoon (not very good but he tries). He only has two small milk drinks a day so he is not filling up on that. He is otherwise generally happy and into everything. I just worry every day as he is so lean he can't really afford to not eat well. Any suggestions on foods to try or basically any information or advice would be appreciated. I am tired of the clinic nurse quizzing me about what he eats and giving me a look that makes me feel like I have failed him, has made me stop weighing him there (I get an approx. weight at home) PLEASE HELP! Very worried mum.