Welcome, Stiff! I am so sorry that you are disappointed in your experience as a MIL and a GM. It sounds to me like it is mostly a case of "mismatched expectations." I take it that you feel you should be able to visit often, including being invited to come in when you just drop over. But it seems that DS and DIL feel visits need to be less often (maybe b/c they have other things going on - visits w/ her relatives, socializing with friends, play dates for the kids, etc?). And that they do not care for drop-ins. Some people don't (I'm one of them). IMO (in my opinion), refusing to ask you in when you just pop over is a clear sign that they don't want anyone dropping in. (I hope you and DH have stopped doing that.) Frustrating, I know! But if you go along w/ the visits the way they prefer, I think you'll be ok.
Obviously IDKY the visits have become less frequent. Perhaps it's just that they have gotten busier with other things. As kids get older, extracurricular activities often increase, etc. which often means less time for GPs. Could this be what has happened?
Then again, perhaps you and/or DH have questioned/argued about the way they're handling things? Pushed for more time w/ them or the DGC? Or continued to just "pop by," now and then, even though they don't seem enthusiastic about that? NOT accusing you of any of this. But IF you and DH have been doing these things, that may be why they seem to be pushing you further away/cutting back on visits. IF this is the case, please stop.
Whatever the reason for the change, I wish DS would be honest w/ you. But since he brushed your questions off, I would take Smileless' advice and stop asking. Just keep enjoying the DGC when you get to see them and avoid doing things you know DS and DIL don't like (e.g. the drop-ins).
Since you still see the DGC, I wouldn't cut DS out of your will just yet, if ever. Later, that will just make them think they should have cut you off completely. Besides, some of the money you leave them may be used to your DGC's benefit. Also, hopefully, you will live a long time, and your will won't come into play for many, many years.