I'm assuming you live somewhere warm enough to grow this tree outdoors, all year round.
To thicken the trunk, allow it to grow out, then periodically cut it back. You need growth to thicken the trunk. I would let it get about twice the height and width that you would desire for the "finished tree". Then cut it back to about half the size you would like. Then let it grow out again to about twice the size that you would ultimately like. Keep repeating the cycles until you have the diameter trunk you would like.