@Joseph CAOA ,
I can relate to this situation on many levels,
The life histories of dating older men (or rather, people with a deeply sentimental history) will always be a bit of a winding topic depending on the level of attachments they hold to their past. I've taken the time to survey and note a lot of relevancies in the many conversations i've had in this topic from many perspectives.
an ex-lover of 24 years is not something that can be let go or muted so easily. the focal point here is the 24 years of your lover's personal life that he lived with this specific person. For most of us, that's more than a quarter of our respectively short life spans. I think it's fine/fair/normal for you to feel insecurities towards his ex lover, especially since they have "obligatory" ties to each other that keeps them relevant in each other's lives, let alone having an adopted child to toss into the mix.
I can understand how tough the situation can be for you on those rough days/nights, and i think your trust and security can only be met not just in humbling yourself to who or what this ex deeply means for your boyfriend, but also your boyfriend taking the time to express to you some empathy and tenderness to allow you to feel more secure as you get "better accustomed" to the baggage/things that he knowingly carried before opening himself up to a new romantic partner.
You guys have been together for (almost) three years now and i believe that says something about what you both have for each other. I do not know the full details of your lives, but i believe him comparing 24 years to 24 months, is him being honest to the realities of what his history entails in relations to you being the latest addition to this aspect of his life.
I have an ex-lover for almost four years, who constantly challenged me from spending time with my closest friends whom i had grown up with, to the point where he would "unconsciously declare" that he should be much more important than they should be in my life now that I was his boyfriend. While that may sound possessive on his end, because ultimately I believe it was, it also made me see what the worth of a proper balance between building life with a new lover while still staying true to the pivotal pieces of your past. And if a new lover challenges all that you had lived through and made of yourself in your past, then i think it's time for an open and honest chat to find a compromise to what it is that's really off putting the other partner, and move forward accepting the realities that cannot be changed between both parties.
Thank you for your reply.