I can't help you with his tombstone, but I know how you feel. My lab had just turned 14 when I had her put down... I probably waited too long but wanted to spend one last Christmas with her. When the time came I told her to get in the truck but she wouldn't do it... although she had arthritis in 4 legs we walked the 1/4 mile frontage of my property one last time.
When the time is right you'll get another. 14 months had passed, I was planning on taking a week off and wishing that I had a dog when a coworker showed up with an underfed, very timid lab who had spent the first year of his life tied out and was being treated for lungworm (apparently from eating slugs to survive) and asked if I could give him a home. That was >3 years ago; we changed his name from Harley to Ruger, who is no longer underfed, and he's certainly NOT timid.