Remember he is 14. Growth can be crazy then. IIRC, I was 1.50m/47kg at 12 and 1.80m/67kg at 16, and most of the growth was around 15. Was it all muscle? Of course not - but very little was fat, you could see my ribs.
Just natural development of an adolescent can put on lots of weight, I was skinny but there were plenty of big guys around.
Brian, if during this time of great natural growth you have a decent fitness and strength programme, and combine it with eating lots of good food, you will build a solid foundation for your adult physique. Just don't focus on particular goals like 10 or 8 or 12kg of muscle growth. Your normal ups and downs of growing up mean that's impossible to aim at that precisely.
Eat lots of fresh fruit and vegies, nuts and beans, drink plenty of milk, eat those big pasta, potato, rice or noodle meals your family is probably eating, lift weights a few times a week, go for runs/cycles a few times a week, go to bed and get up early, and you should be right. Enjoy the journey.