If your horse has western pleasure training he likely just needs to be reminded. In a couple of years they can get pretty rusty so start from scratch with him.
When you ask him to lope off does he accelerate (get faster and faster until he is more of an English Canter?) If so simply ask him to lope, sit nice and deep in the saddle, and if at stride 4 his lope is too fast stop him, relaxe him and ask him to lope off again until he gets too fast. The key is to make him realize that speeding up is going to equal a stop, and a restart which is very hard work for him.
As he gets better and he gets even a little rushed you can circle him back in a tight circle and that will slow him up. If it doesnt go back to the stop and restart until he gets it right again.
Welcome to the forum and best of luck.