I am incredibly sorry for the loss of your son.

Have either of you gone to therapy in dealing with this loss? The sudden loss of a child takes its toll physically and mentally on the parents. You are each a reminder of this time in your lives. And if you don't address it in a healthy way you both may end up misplacing your grief and creating resentment towards one another. If you haven't sought therapy I definately recommend it. Individual therapy for you both.

As for the relationship, take it slow. Date each other again. You are both different people now. Things aren't the same from when you met. Your experiences with each other and with life have changed you both. In personality, in interests, in defences, and in coping mechanisms.

Go on dates. Learn about each other again. Dont rush into rekindling the engagement. Take this time to explore the relationship. Its awkward for now. Maybe it'll stay that way. Maybe it won't. But communicate and let it flow naturally. Don't try to force yourselves "back on track" because that track doesn't exist anymore. This is an entirely fresh start.