How green is this horse? 99.9% of the time a horse will naturally be more comfortable and balanced on one side and they will only want to pick up that one lead. How I get a young horse to pick up the correct lead is to work in a ton of circles. I start them out in a fairly large circle at a trot, then I make sure that I am balanced and cue for the lope. If they pick up the wrong lead, I tighten down the circle to the point where they are uncomfortable (unfortunately, you will be too) and keep them loping. Once they start to get tired, it doesn't take long, they will often change leads to the correct lead. They may crossfire for a few strides or drop down to a trot. If they trot, then just balance up and cue them for the lope again. Once they pick up the correct lead, then let them relax out into the larger circle for a few circles and then slow it back down to the trot or walk and do it again. It is really important to not overdo it because if a horse is not balanced or comfortable on a certain lead, then it is hard on them to work in it for a long period of time. It may take a few days for him to become consistent at picking up the correct lead when you cue for it but when he does, praise him by letting him lope a few strides then drop back to the walk and walk a few circles. The more comfortable and balanced he gets, the longer you can ask him to lope. Don't get impatient or frustrated with him, that is just something that takes time.