There are a few things you could try.
First you would need to split up these two in order for him to be available to return to you, so for this I recommend a vinegar jar spell. This will make their relationship bitter and sour, causing them to fight and eventually break up.
Then to get him back, I suggest a honey jar spell, this will sweeten him towards you and enhance already existing feelings. It's good that you are still close to him as friends, because it makes a lot easier than someone who currently isn't talking to their ex, plus you can get personal concerns from him such as hair, nail clippings, bodily fluids etc a lot easier, which you will need for this kind of spell.