I found an old birthday card of my husbands when his mom passed. I got a heart shaped key ring that had “lots of love, mom x” in her hand writing.
When my step daughter’s mom died I purchased a tree through trees for a change. They plant trees in honor of a loved one in areas affected by wildfires. They send you a certificate, and location of where your tree was planted. So we can go for a road trip and visit. This way her mother’s memory will be here long after we are gone. My step daughter is big on the environment so this was something she absolutely cherished.
Etsy has tons of memorial stuff available. Just seeing my dads picture in the house gave me comfort
If he is more of a quality time type person I'd honestly just offer to go on a walk together. Sometimes being outdoors and spending it with the only person you find comfort in can mean a lot. My husband is a walk type of quality time and it can be so refreshing and help relax and clear the mind a bit.
Making a framed photograph of a family photo. A cherished article of clothing made into clothes for a stuffed animal of your child or children. Make his grandfather's favorite dish or going to his favorite restaurant.