From what I can see the tree is very young, focus on a good fertilizing routine for the next few seasons, check the rootball, if its pot bound you can transfer the tree to a larger pot in january, it needs to grow more if you want it to become a nice bonsai, some others with more pine experience might also hhave advice, while its growing learn all about white pine.
I would just wire the side branches down to let more light in. That will help it back bud and open you up to more choices in the future.
Only height will give you thickening ....
Here we go again...Sorry but I disagree. It may be the fastest way but not the only way.
If you are satisfied with the size of the trunk you can start developing branches. Normally branches are cut to a small shoot if the size needs to be reduced and ramification started.
At Taisho en they have been doing experiments for 3 years now with older pines by removing buds in order to promote back budding. Some branches back bud and some dont. But generally the results are positive.
Here is a tree from this experiment.
Here are the buds removed:
View attachment 45873
and here it is buck budding:
View attachment 45874
Hope it helps.
Indeed didn't meant to use the word only that literally.
However growing in full ground conditions may it be a pond basket or whatever, can also help you thicken the trunk using a sucker at the base of the tree. This will thicken the tree at that point after a while but they mostly are so close to the base that they need to be removed.
This will leave a big scar or jin.
But also length growth is needed to thicken the whole trunk even and crate some taper.
General rule in trees in general is the bigger they get the thicker trunks they get.
Since pines are apical dominant they need a bigger trunk to send all those resources up the ever growing top that wants to go high.
If growth in the top is limited spread over several branches trunks don't thicken much and flow of nutrition is balanced.
Thats why they develop the weaker lower branches first while letting the top shoot sky high and if that is done, cut off the top growing branch and form an apex out of smaller branches.
The small shohin black pines with thick trunks and dense foliage you see at nursery's around the world. And will cost you hundreds of dollars.
These little ones were seedlings or cuttings cultivated like this for about 7 to 10 years.
Takes time but time is the only way to make good bonsai.
Only height will give you thickening ...
Here we go again...
It is pointless getting hung up on the word "only" when his post is 100% right. If it's not the only way, add something worthwhile by sharing your experience thickening the trunk of your JWP.
I let mine grow every direction, and that includes getting taller...as trees tend to do.
If you don't work on the branches you want to keep they will become pom poems because it's already been said that pines are apically dominant meaning they want to grow at the top and the ends of the branches.