Yeah I can see how it'd be uncomfortable, especially when it's DH's boss
Before DH and I decided to start TTC, one of his friends (he has 2 kids) was constantly commenting my "flat" belly every time we saw him
I never knew what to say so I just never said anything. After we started TTCing, another of his friends and his wife who have their first child have been pests, mostly the wife. I remember at one get together she was cuddling with her little boy and saying "don't you want one of these?" to me. I just didn't know what to say as A) it's none of her business that DH and I were TTCing and B) I don't know her well enough to get into discussing the fact that DH and I were (and still are) having huge problems conceiving. So I said nothing and just let there be an uncomfortable silence. If it happens again, maybe I'll try and start crying and walk out so she learns that it can be a sensitive question for some and she'll learn to keep her mouth shut
In hindsight, maybe it's best to just say something like "We'll see" and then turn the conversation and start talking about something that interests him like maybe if you know his boss' hobbies or maybe just talk about the company or something. You could also just not answer, smile, and just change the topic and talk about something else. If he insists, just say you don't want to discuss this topic with him and then change the topic to something else. If you're into feeling more gutsy, maybe "Wow that's a very personal question, don't you think? So how's it going with...." If you keep saying you're not willing to discuss it with him and he keeps going, just walk away. He can't possibly get mad at you for refusing to discuss something you feel is personal and don't want to talk about.
Frankly I find it odd he's asking you and not your DH (it'd still be rude but he knows your DH better than he knows you)
But maybe he has some strange notion that it's always the woman who is dragging on the process or something
When it comes down to it, asking someone when they're going to have kids is a rude question
If those being asked don't want any, they're put in the uncomfortable position of having to defend themselves and infertile couples are just upset by this line of questioning. I've always found it rude, even before mine and DH's fertility issues. I would never ask
anyone this question as it's none of my business, even if it was family.
Hope you get the boss to leave you alone next time