And to get that trunk you want you will have to let this tree grow....a lot....and very tall. Then you'll chop it back. Or, as
@sorce suggests, look for a bigger tree to start with or ask the dad landscaper for a bigger tree. It may be unlikely that he has Elm trees...but may know an area where there are some growing...or be able to offer you a healthy substitute tree.
I believe that keeping a stick in a pot and letting it get root bound will result in a root-bound-stick in a pot just a little bit thicker.
This is what I do.....
All my American Elms started as sticks with about a trunk diameter of a whopping 1/8". I take these sticks and first free grow some more roots in a plastic nursery container. I let the trees grow...totally untouched for at least a full season and sometimes two seasons. Then I pull the trees out in spring at bud swelling time and prune off any tap root and any undesirable roots. I root prune. I now move the tree, which probably has a trunk of 1/2" at least into a wooden box. I use a box about 15" x 11" x 4.5" with slats for the bottom. The tree will stay in the box to free-grow again. Sometimes, when I put trees into the box, I chop them back....just because they get to tall and because I want a directional curve in the trunk....and sometimes just because I just need a tree to chop. This early the roots are to young to tie down. I just place baseball size flatter shaped rocks on the surface around the trunk....to keep the tree secure until the roots take a strong hold in the container. It has worked for me...nothing glamorous...just functional.
After the box....I move the very healthy growing tree to the ground. I don't have any clay....excellent! Just a sandy soil so I add some old bonsai soil to the ground area where I'm planting. Once in the ground, planted on a tile (what I use is 16" x 16" floor tiles), the tree will grow...large...fairly quick. I arrange the roots. In 3 growing seasons I can go from a 1/2" trunk to a 1 1/2" trunk. Every three years I trench around the tile shape ( I mark corners with stakes)....but I just leave the tree in the ground. I can't seem to resist the chopping of unwanted wild growth pattern branches....I have to work on letting growth just happen....my chopping seems to slow growth down....but at the same time it does provide movement in the trunk.
Fast forward. About 7 years later I have a decent gnarly trunk with healed scars of about 3.5 "to 4" diameter. I also have reasonable taper developed. Now I'm chopping again and beginning branch development....I hope.
Basically....if you want a thick trunk....I'd find a 3' circle to grow your current stick tree in the ground. And when the tree is tall....cut some branches off and put those new sticks in a plastic container. Water well....grow more trees to turn into whatever shapes you might want to try. Plant them straight...plant them slanted. Chop them. Grow them. Have fun learning how the trees respond to your work. And...watch your dad chuckle and shake his head left and right as he examines your efforts. But...you have a plan.
That's what I do.
As
@sorce says though...find a thicker tree to start with and save a bunch of growing years. I choose the growing path because I like that way....I enjoy the growing....and nature is a great helper. But I'd still grow the tree you have out...gives your dad some free joke material for conversations.