Cool kid, go Janek!
In the video it's obvious that his racquet is slowing him down in almost all situations, too heavy, or too heavy in the head, or both. He can't easily do the serve movement, and in points if he doesn't have enough time to transfer all of his mass into the ball, together with a super wide swing, his balls are short. It means he has to backtrack often just to have a step forward into the ball to be able to transfer his body mass, typical for kids his age. And if he can't take that step back because he has no time in the point then his swings are too weak. He also can't line up shots nicely with it, meaning he doesn't centre the ball for a clean power hit, because it's too heavy and wobbles in his hand, so he's brushing up to control for off centre hits, which makes him lose even more depth. Other boys in the video are swinging much quicker, it's easy to see.
It's very easy, and a very common mistake when testing racquets, on all levels, to feed easy balls and see how the racquet swings and hits. It's easy to line up and centre the ball, heavy will always feel better than light in that example. But to really get the feel for a racquet's downsides you should just do points play, just play points with underhand serve, stretch him around and test yourself how he hits with a racquet when under pressure, when he's late to the ball. Give him one deep ball to make him backtrack, then a junkball into the service box, left fast flat, then right spinny high and slow etc, and you'll quickly see with which racquet he's on time, or always late, and if his balls are falling too short and weak, you won't even need to ask him. With a lighter racquet he'll be able to centre the ball and get a clean power hit way more often in points play, he'll be able to take initiative.
Heavy doesn't help at that age, 99% of balls they hit in a match are floaters, speed matters, both in legs and feet placement, and in swings and racquet head speed. With a quicker racquet he'll be on time more of the time, and will more easily generate ball speed and depth from a slow floating ball. It's a simple logic. There's a reason why all top new gen ATP pros play with light racquets, around 305g unstrung. Fritz even plays with 295g unstrung, just read it today on this forum, sw of 330 strung. So basically 5g lighter than new Speed MP he tested, with a same or similar swingweight. Fritz is definitely not a quick mover, it's easily his biggest problem in tennis, so a light racquet makes a lot of sense for him.
Heavier/sw racquet helps to block if your opponent is blasting rockets at you, which doesn't happen until 15-16 yo. And again, testosterone will start only at 14,5-15 years of age, then muscles will get more explosive, there won't be any meaningful real power before that, all he has is quick feet, good anticipation and speed speed speed. Heavy = slow. Takes too much time to swing, and move his feet both before and after.
With Extreme Tour all this heavy, weak and late will happen much more. Why don't you test L versions, Speed MP L, Radical MP L? Those will have a 315 swingweight, and if he needs some counterbalancing it's super easy to add silicone or lead to the handle. He can easily grow with them, it's easy to add 2g at the top later and compare. All Head pro stock racquets, made for pro players, come in super light form, 270-280g I think, so lead and silicone are added later, more than 50g in some cases.
If you strung Lynx Tour fresh before those matches then that's a big issue also. Too low powered string bed makes him try to hit too hard just to get depth. Why not just Lynx 1.25 at that age? Or at least Hawk Touch 1.25, LT is too low powered for such a young kid
Seeing how he plays, he likes to attack the ball, which is really great to see, congrats to the coach!, new Speed MP L would probably suit him best. Last two gens of Speed MP were launchy, too high ball trajectory, needed a lot of spin to control for that. And since now Sinner is a superstar, and the face on the cardboard on new Speed racquets, so this new Speed MP is totally the opposite, lower trajectory flatter hitting racquet, made for all the kids who want to be like Jannik. Maybe doesn't seem great initially when testing, but will definitely force your son to attack and hit through the ball, and it's still a 23mm 100 so it's forgiving enough, and he can dig himself out of corners easily. And try it with Hawk Touch, it has ample snapback to give more launch and spin and better trajectory. Lynx too, both will be better than LT for him.
