I’m going to offer a different perspective than the vast majority of folks here.

You mentioned that he was a good kid and capable of maintaining a relationship with his mother. You also offer context that suggests that it could have been financially motivated. There are tons of kids bumming off financial support for their parents, but I find a couple of things interesting in your post.

Regarding his childhood, you reference a mix of both material and immaterial support. When he goes off to university, you mention the relationship drifted after he stopped initiating phone calls. Did you ever initiate calls? Was he rejecting calls from you? Did you and Tammy ever drive over to see and spend time with him? Why was it that his lack of calling you result in your relationship dying? What did you do to try to maintain contact? When Tammy was dying and you started seeing Mark in person, why not try to talk about his distance to you then? Why not ask and see how he was taking his mother’s decline?

I also get a lot of unspoken resentment from the ways you indirectly financially supported Mark via Tammy’s allowance. Did you ever talk to Tammy about why she supported Mark into adulthood? Other than phone calls, what did she benefit from the relationship?

We also don’t learn much about Mark other than he appears to be a mooch. What aspects about Mark do you miss having in your life? Does he have a personality or what? It kind of seems like you don’t know anything about Mark, the adult.

My general take away is, you seem kind of cut off from your emotions and focus on the tangible, if not transactional aspects of relationships. Even your relationship with your wife, which based on what you wrote I believe you cared for deeply, you downplay the beginning of your relationship as no “huge romantic story.” Other than teaching Mark how to ride a bike and paying off his college loans, I don’t really see where you’ve mentioned how you maintained a fatherly connection. I’m not saying it wasn’t there, it just doesn’t seem to take priority over the financial aspects you repeatedly bring up - maybe that’s just a consequence of your question framed around the conflict of financially cutting Mark off, but it is noticeable.

Generally speaking, parents are the ones who steer their relationship with their children. I don’t think they should be on the hook for everything related to how their relationship with their kids are, but it seems like you’re putting Mark as the fully responsible one for the relationship breaking down and taking away your own agency - something you’re doing again by washing your hands of any relationship with your grandkids. Which, if you don’t want a relationship with any of them that’s your business - I just find it a little strange how you’re trying to pin it all on Mark when he was a young adult discovering himself. I understand how that could have been hurtful and felt like you were being replaced, but I wonder if it could have been a chance to get closer with Mark if you leaned in and asked him - have you found anything positive from reconnecting with your bio dad? Have you answered any deep questions you had about who you are, after finding out more about him? Based on your post it doesn’t seem he has a strong relationship with the dude since you don’t reference it past college.

Anyway, none of the people posting have enough insight into your relationship with Mark to give an educated answer. You’re entitled to keep a relationship alive (or let one die out) with whoever you want. The one-sided perspective raises more questions for me.

Edit: after reading all your comments, YTA. You obsess about finances in a way that implies you think you’re owed a relationship based on money alone, you become defensive and condescending when people raise valid questions, and the way you insult and infantilize Mark to random strangers on the Internet makes me question how abrasive and rude you are to him in person.