Yorkie body types?
 

Draco is now 6 months, this supposedly around the time where he'll stop growing but may fill out a bit more which is what I'm hoping. He's 5.5lbs and 10in long(neck to tail) and just at 9in height(floor to shoulder). He does feel super sturdy nor super fragile, though he already broke a paw at 3 1/2months which has made me even more cautious.

Is it possible to help him have a sturdier build? I know it has a lot to do with bone structure but maybe helping him have stronger muscles? He swims in a dog day care every other week for 20-30min. (he still doesn't care for the water lol) But maybe longer fast paced walks could help?

I keep seeing the forums that jumping is bad, well he was discouraged jumping on people and doesn't do it too much. But it doesn't keep him from bouncing around and following me around the house. He's filled with energy, and when he gets the zoomies his route is around the table jump on the couch then of then back around the table. My couch is pretty low at 6in off the ground and about 6in thick so I don't feel it's a big deal. Also I've wondered about putting him in agility when he's done growing just for fun. I just want his body to match his spirit as much as possible, not constantly restricting him.

Ha-Draco has the same zoomies path as Huey...around the table, across the sofa, launch self across room, back around table, etc. It is heart dropping to see them jump like that. I pray for a good landing every time!
I think the swimming is an excellent idea to increase strength. Some dogs are just finer-boned than others. Agility sounds great, too. Many dogs on this site do agility trials and do very well, beating larger and stronger dogs.
There is also the Earthdog trials if he is a big digger or hunter.
I believe he has to be AKC registered for actual competitions for both.

Give him time to develop. At 6 months, he is still a pup. Max developed in his second year, and Teddy in his third year (late bloomer). Teddy was scary skinny his first year. He has a very fine structure, but he finally built chest and leg muscles.