I agree with what people have said so far, but I just thought I should add that when you do take him out on his own, take him out when no other horses are out on rides if possible< I have an 18 year old gelding that will still race around out on a ride if he knows another horse from his field is out on a ride and try to catch up with them, which isnt such a pleasant experience as he would probably canter/gallop the majority of the ride to catch up. I would also make sure that you have another rider (if possible, just any person if not) to walk with you to keep your horse company and 'babysit' them.

You could start by increasing the distance between you and the horse with you. Stay infront, so he can't try to catch the other horse up. After you can be quite far away, try going out of sight of the other horse, like turning a corner, so the other horse will be back in sight quickly. Gradually increase the time he can't see the other horse, until he doesn't notice when their gone anymore and then try hacking on your own. Take someone on the ground with you if possible, maybe with a lead rope incase you need abit of help. Having the lead rope on without the person controling him at all might make him think he has to behave.

Remember that you have to make him confident, take things slowly and only do what I suggest if it's the right thing for your horse, other methods could be better and rememeber to stay safe if you're on a busy road on your own! Hope it goes well

The way I got my mare to hack out alone was to start by long reining her, in an enclosed space to start with and then out on the lanes.  This really helped her go forward and she was soon responding to my voice rather than any pressure on the long reins.  After that we progressed to hacking out with someone on the ground, sometimes beside us and sometimes behind us.  This kept me calm and helped my confidence which in turn helped her.  Then we had our first hack out alone and it was brilliant, a couple of wee spooks but we had a lovely time and have continued to do so.  I think the best thing to do is not rush things but to keep the experiences positive and not be afraid to go back a step to restore any loss of confidence.  good luck