In theory, TSA and FAA rules allow batteries up to 100 watt-hours to be carried on as hand luggage “if suitably packaged and protected”.
In reality, you’re just asking for trouble. ESPECIALLY during the busy holiday period.
As always, with any specialty/sporting items, it’s not what the rules allow. It’s down to what the individual TSA agent knows ABOUT THEIR OWN RULES.
Once you step outside of the usual situations with liquids, knives, nail clippers, etc, etc, you’re on shaky and highly-variable ground as far as TSA agents really knowing and applying correctly their own rules. Now you throw in the fact that the jump pack will be rated in AMP hours, NOT watt hours as the TSA rules state, and you have to have an agent that can do a conversion (BTW, divide watt-hours by voltage to get amp hours).
So you see the problems? Add in that it’s one of the busiest travel times of the year, long lines, the labor shortage, etc, etc, etc and what are the chances your son will get the TSA C-team that doesn’t know the rules or how to use them?
Why make this harder than it needs to be? It’s a great gift idea, BTW. Send the gift to his home and get him another small gift to enjoy while he’s here.