This is an excellent explanation for someone of his age and his skill with video editing obviously helps a lot. It held my interest, the world needs more entertaining and educating videos like these.

My only criticism - and some youtubers have already pointed this out - is that the explanation of time dilation "..the same bodily change that happens on earth takes much longer to occur when you are moving so fast.." is wrong.

Signals sent within the body can be analogous to a clock - any fixed duration measured between two ~lightspeed reference frames will be different, including seconds measured by an atomic clock - but time dilation specifically has nothing to do with the mechanics behind how you measure the time or the time it takes a signal to travel. It's a property of the nature of spacetime. Time itself actually slows down. There's no 'trick' to understanding how or why, it's just a property that it has. We can forgive him because he'd already demonstrated that physics is the same in any inertial reference frame and there is no "preferential" reference frame; therefore the motion of the reference frame can't be responsible for the observed difference, so he obviously already really knew all this.

There's no shame in getting that wrong, because he'll be taught more and better about it as he progresses through school. Generally the arbitrary subjects are the hardest to live with because you just have to accept them as they are rather than 'understand'. Quantum mechanics is the same - you just have to accept the rules and apply the maths. Everyone struggles with it, even Feynman said "If you think you understand quantum mechanics, you don't understand quantum mechanics."