My mom has an Icelandic Horse that she has sent to me for some 'tuning up' while she is trying to sell him. He has not been advertised as professionally trained, and the ad mentions that he tolts freely in the field, but is just beginning to work on it under saddle. I am not a professional trainer, but do have good results with refining cues and getting a horse to be more responsive, so I'm spending some time on him. His halt is coming along nicely, and his walk, trot and canter are quite good, right lead needs a little work, but we're working on it. His tolt, however, I can only get for a few strides.
I had spoken with the owner of a large icelandic breeding/training facility a few years ago when we were having trouble keepimg his weight up, and she told me to push him forward with seat and legs from the walk while holding him in tightly with the reins and not allowing him to trot. It is gradually seeming to work, however I am not sure that this is really the best way to go about it. I feel like I'm having to hold him much too tightly to keep him from trotting, and sometimes he just goes into a very slow lope instead of tolting or trotting.

I am currently riding him in an eggbutt snaffle, as I am most concerned with improving his stop, and would like to use as little bit as possible to achieve this. Is it possible that I'd have better results getting the tolt with a stronger bit (he was regularly ridden in a Kimberwicke with a jointed moutpiece until I started working with him)? Is there some other method that will be more clear to him?
He is a fantastic trail horse who will go anywhere, and do anything you ask, and that's what we're selling him as, but the tolt is important and we know he can do it.... There are several people interested in him, so it may be a moot point if someone snaps him up right away, but if not, I'd like to see him make some progress.