I rather like the knot in your avatar.
Is that too much knot work?

Any kids? I ask because maybe a solid cross thru the middle with a bit of knot work surrounding birthstones around the solid cross. Or maybe just birthstones intertwined with knot work without the solid cross in the middle. I think I’ve seen a Celtic cross with a heart in the middle. Maybe have some birthstones surrounding a heart.

No kids or fondness of the stones...maybe a mix of something modern that peaks his interest like relating to a hobby or a symbol of something that has touched his heart/life (other than you of course).