I always teach lateral work in hand first - it can take a little practice to coordinate yourself and make yourself understood but persevere. Once you've established the leg yield it is a great tool for helping make your horse more supple through transitions and the ability to yield to the inside leg is essential for straightness, collection and contact....it is all part of the journey to 'thoroughness'
Please remember - this horse is only just 4 so he is right at the beginning of his ridden journey.