“My partner doesn’t seem to reach out to connect to me, and as a result I don’t feel we have as strong a relationship as we could.  For example, I'll ask him tons of questions about his childhood and he doesn't even care to say, "What about you?" Is that common for ADHD? He is the kindest man I know, but his lack of curiosity feels depressing to me.”

There are many things that could be going on here – from anger or a sense that connecting with a partner feels unsafe, to distraction, to a simple lack of awareness.  I’m going to answer by assuming that the relationship is relatively calm, but that the ADHD  partner is not tuned into the desires for greater connection that the other partner holds.

Those with ADHD may be socially awkward or seem to lack curiosity about their partners.  Further, research suggests that people with ADHD have difficulty reading emotional cues from others.  So, you might be exuding something like "I really want to engage" and he might not sense that.  In addition, my observation is that many men have not been taught to delve deeply into emotions with their partners.  This is a skills issue, largely based in an American social construct that says that men should suck it up and deal with stuff and that showing or exploring emotions may be a sign of weakness, vs. women who are encouraged to explore the world of emotions and are given lots of practice in those skills.

My suggests are these:

• Don't expect your ADHD partner to lead on emotional issues or to anticipate what your emotions are or when he should respond.  Instead, invite him into emotional conversations overtly.  Look for the ability to respond to your invitation as the goal.  So, for example, after you’ve asked him about his family, you could transition to talking about your own family by saying “That makes me think of my own family situation…” or “Now that I’ve asked you a lot of questions about your family, what questions do you have about mine?”
• If he has trouble responding or being able to talk about his own feelings within the context of a conversation, invite him to work on better understanding his feelings and using words to explain them.  Be patient, open and inviting.  Don't judge him for using words you might not choose or for being slow to be able to name what he's feeling.  Ask him to look underneath his initial ideas, perhaps with the simply question "what's underneath that feeling?"  Responding to and verbalizing emotions takes practice.
• Respect that your partner is the expert in what he's thinking/feeling.  And that he may well not talk about it the way you would.  If you don't understand where he's coming from, ask questions, but don't correct or try to 'steer' how he feels to something that feels more comfortable for you.  His feelings and ideas are his own.
• Your partner may not feel a spontaneous need to connect, particularly verbally.  But that doesn't mean he can't responsively connect with you.  Look for ways for connection to feel safe to you both, as well as making interacting appealing with positive feedback, invitations to connect, and planning times when you can have fun doing things together ('fun' seems a particularly good way to start the reconnection process with partners who have ADHD.)  Rebuilding connections when there have been relationship issues take rebuilding trust and calming the relationship...so it can take time.  But if your husband is 'the kindest man you know' then you both have a lot to work with and admire.  Open up and see if you can have some fun with him.

And remember, there are many ways to connect and know you are loved.  Make sure to create time in your relationship for 'attend time' (time when you are focused only on each other, with no other distractions); for having fun and 'playing' together; and for sharing all that you are grateful for with each other.