You can't keep your husband from having sad memories or thoughts on Father's Day and I wouldn't try to stop that. He grieves however he grieves and unless he's harming himself and/or others then there's no wrong way, you know?
When my husband's brother passed away it was devastating to the family and in particular to my husband. The fallout affected our marriage because of how much emotional turmoil he was in over the loss of his brother. All I could do was let him talk when he was ready, let him vent, rant, work things through. I let him know I was there to talk to and let him cry if he needed to. He still managed to misinterpret, misunderstand, and misremember things I said and did because his grief was so profound but we worked through it. After ten years, though, he still can't talk about his brother without crying and romanticizing and blame (for his brother's condition which was a lifelong degenerative neurological disorder).
But now his father is in a nursing home receiving palliative care and after a few months of very rapid decline is still slowly dying and shutting down. My husband is already grieving for his beloved father and feeling overwhelming guilt. We talk, and I try to maintain talking about H. memories and strengths his father has always had. I don't need for my husband to say out loud how much he hurts about his dad or his brother because I KNOW. So I hug him, I'm here for him, and if a subject comes up naturally I just go with it and let him lead the rest.
As for Father's Day, maybe you can print out a nice photo of his father and overlay the Foot Prints In The Sand prayer onto it. My SIL did that for my MIL (translated into Italian) when my BIL died and she made one for everyone. It's one of my husband's most prized possessions.