I've got a Mitragyna speciosa sapling which is all "leggy" stems with few leaves. 

How to I stimulate this sapling to sprout more branches as well as to stimulate leaf production?

 

Looking at the photos, I would say the surrounding shade is the problem, like that big tree, it is short on space it is getting leggy as it is trying to get more light by growing up, and not putting out any new shoots.

Kratom may not grow as well in northern Thailand as it does at our daughter's farm in Chumphon, where it thrives even in partial shade of larger trees.

 

Give it the best growing conditions, soil and water management. Soil heath building with bio-fertilizer, bio-stimulants, mulching and deep watering once or twice a week during the dry season. 

 

I don't grow kratom, but for other tree and landscape plantings and veggie garden, I use biological fertilizer,

- a bat-guano/cow-manure product available at an ag shop across the road from MaeJo University.

- Takumi Fish Fertilizer, also available in Mae Jo, at the Maruchu Bussan store.

I also use a biostimulant, liquid worm castings effluent, from MJU cannabis farm vermicompost store. 

 

https://www.fertileearthlandcare.com/watering-trees/

 

Yeah - the sun is an issue.  It's got a lamyai tree branch right over it.  I'm going to remove that branch later in the year, but I've bought two more sapling that I've planted in the full sun. 

I've also clipped the leaves at the ends of some of the branches and the branches I've done that too that get the most sun have begun to develop more branches.  Also using a 15-15-15 fertilizer in conservative amounts weekly.  Seems to work.  I've asked the wife to get me some ox poo but she never gets around to it.  I'll check the local farm shops for that other types of fertilizer.  And I've got a good supply of mulch and compose so I'll work on soil quality as well.