I have a 7 year old male. Weighs 85 lbs. he's always been skinny. I wouldn't worry about it. He poops twice a day. When he was between 3 and 6 years old and much more athletic, he topped out at 90. That was the max weight ever regardless of how much or what he ate.
You could try coconut oil which Traveler loves and/or Missing Link which is a food supplement that contains all kinds of good stuff. That puts weight on Traveler.