It was our sons first birthday on Tuesday. He is the first grandchild to both mine and DP's parents. Thing is, DP hasn't heard from his dad at all this month. He lives in the same town as us so you can't blame it on distance.
Although he tries to hide it, I know DP is upset that his dad had forgotten DP's birthday. He did mention it last time they spoke, about how we weren't doing anything extravagant, just a mini tea party at ours.
He is a bit somewhat...uninterested in DS. He was very supportive during the pregnancy, would invite us over regularly, offer to help with things we might need for the house as well as baby. Then as soon as DS arrived, it seemed like he had lost all interest 
I'm not saying I expected anything from him for DS' birthday but a courtesy phonecall would have been nice. Especially for DP who feels his dad never makes an effort.
We bumped into his dads OH in the supermarket yesterday. I was tempted to make a remark about how fab DS' birthday was after she asked how we were but it's not her fault her partner is so slack with his son and grandson. DP's brothers have, throughout my sons short life, mentioned to their dad about his lack of effort with DP and DS but it hasn't made any difference.
I'm not saying I think he should make more effort, it's his decision at the end but would it be unreasonable of me to send a text saying 'get in touch with DP...he is a bit put out you forgot DS' birthday'?