The women you are describing sound to me like they have lived confused, chaotic lives themselves and simply do not understand limits or 'normal' family interactions. My mom was raised in a certain degree of havoc – a dad who had to go into hiding over gambling debts, the Great Depression and her mom having to scramble for odd jobs to feed her children, and much more.
My mom grew up with some very strange ideas of her own about how not to let her family go the same route, and created a whole new layer of dysfunction for me and my 3 sisters. So by the time I married, I was pretty sure I wouldn't have kids. I knew that I didn't understand 'normal,' and was afraid I'd bequeath chaos and confusion on another generation.
I did eventually have a daughter after doing some serious work on my dysfunctions. My mother adored my daughter and tried, in her infuriating ways, to be her mamma. I gradually realized this was her way of trying to make up for the ways she failed me. It was a crappy approach, though, because neither she nor I had healthy boundaries – neither of us had ever seen healthy boundaries.
I'm guessing that your MIL and her mom may be in the same fix. They may simply not know where they end and your son begins. They may be trying, with their excessive gifting, to atone for the past your boyfriend had. At their ages, change will be hard, especially if you are not clear yourself about your own boundaries.
It sounds like you recognize the need, and are infuriated by the lack. But I wonder, K., whether you chose your boyfriend, in part, because of unconscious boundary issues of your own. It's all too true that many women, especially in their younger relationships, seek unknowingly to replicate some family dynamics from their own childhoods, because that's what they know. (I did that with my first husband, and suffered hell for it for 15 years by the time we divorced.)
I doubt that this will sound very satisfying to you, but I would suggest two things – work on your own boundary issues. Find out what they are, how they became part of your life, and learn new ways to set limits for yourself and those around you. It's not easy, but there are some excellent self-help books, counseling, and even Al-Anon groups have been helpful for a couple of acquaintances of mine who could not afford counseling. Al-Anon is really big on forming healthy boundaries.
And until you get that sorted out, do your best to understand that these women are struggling with their own regrets about the past, and desires and hopes for a better tomorrow. In their eyes and hearts, your son may well be a fresh chance to make good. They probably can't see how exasperating this is for you, or understand why you react as strongly as you do (which could be a reflection of your own unresolved boundary issues). Their intentions are probably generous, as are yours in wanting to give those gifts yourself.
Finally, since you know what these women will do if they hear your plans, stop talking about your plans to them!
I could be totally off-base here, because I'm reflecting on what these issues would be about if I were you. But if any of this sounds like a good fit, you at least have a starting point. Blessings.