tips to keep your young walker in gait:
something that I found out while working with a 3 year old walker a while back is keep constant forward pressure with your legs as well as keeping their head in, use half halts to keep his nose in, that will keep some slight pressure on his mouth as well as keeping him rounded and balanced beneath you.
as far as building muscle and keeping him out of the trot, use a field with tall grass, hills and or mountains will help also, that will get him to build hind muscle as well as making him use his butt muscles and gait. it may just be a lack of muscle, or it could be something more.
Good luck! Hope this helps!
16 year old TWH Mare named Ginger
3 year old APHA Paint Gelding named Fox.
RIP Evie (Clydesdale) 7.29.09 & Magic (OTTB) 2.23.10