Is it into his yard at all? My husband is, and I'm going to give him a fruit tree.
You could make him a coupon book with coupons for dates, a movie rental of his choice, a back rub, etc.
When you only have a baby in the house, it's really more about your relationship with your husband than your son's relationship with his dad.
You could make him something to hang on his wall near his desk with your son's hand prints or foot prints on it, or photos of him. Once I took a series of photos of our first son (about 6 months old) wearing a onesie with DH's favorite baseball team, sitting in the grass, holding a baseball bat and baseball. It turned out super cute! I found a photo frame with three small openings and put the photos in there. He still has it at work, even though our son is 6 now.