In our house we have a rule that before a birthday or Christmas, we need to make room for the new stuff by getting rid of stuff. This rule applies to my husband and I as well, because there are 4 of us in 750 square feet.
So rather than amassing, we are replacing.
The beautiful thing about legos is that they are small. And expensive.
The other way to get Legos is with allowance. In saving their $3 of spending money a month, they practice restraint in buying something cheaper but less satisfying. I'm hoping this will eventually lead to delayed gratification in other things.
My girls and I do not want the girly legos especially after I talked to them about why I did not like them. After having this discussion with them I realized one thing I didn't like about many of the collections is how specificaly toward boys or girls they are. I don't like ninjago or playing pink cupcake shop (at least not when there aren't any cupcakes to eat). When I stick to toys with mass appeal in our family we have better luck - I am more willing to play along, greedy monsters are less likey to show up, toys play better together (legos and silkies for example), my children play better together, and we purchase fewer toys.
I kinda think it's awesome your son is into legos and has no exposure to TV.
But I get how limited the sets are and how expensive they can be.
Here's what we've done when there is a toy that my girls want but we don't *agree* with the consumerist attitude that is developing - the entitled to everything emotion.
We tell them, it's OK to get it, but they have to PAY for it. Wow, does that slow it down a lot. My girls have both saved their allowences ($3/week) plus birthday money etc. So they have the money. But when they have to part with it, there is a lot more thought involved. I'd say 75% of the time they decide not to buy the toy in the end.
My girls are 5 and 7.
Another idea: talk to your library about starting an afterschool or camp lego club, like a pp suggested. Go to the thrift store and buy some used, universal legos, or solicit donations through the school. That way it becomes a different sort of activity -- social, and free of charge -- AND good for the community.
I've been thinking more about this, OP. I really see where you're coming from. I share a lot of your values, and when my kids were tots playing with their small collection of wooden blocks and woolen felt food and silk play scarves, I never could have imagined I'd have to deal with this kind of issue. Most MDC moms have very strong social values, and that's a good thing. But unless you're willing to relocate to a completely isolated place in the world, your child is going to have some contact with mainstream values. You can ban media, but unless you want a lonely kid, you can't ban the entire neighborhood. My daughter learned about Barbie dolls from a book she found at the public library. I was inwardly horrified, and more so when she wanted to purchase one with her birthday money. But I knew that saying no would signify 1) that she didn't even have control of her own money; 2) that I was critical of her interests. No way was I going to risk that. She purchased the Barbie, and now has a large collection that brings her much joy. There are things I still don't like about Barbies. But she's gotten a lot out of them, things I wouldn't have expected and couldn't see until I took the time to watch and observe the world through her eyes. I think we can model values to our children and have lots and lots of discussions, but ultimately we have to give children room to find out for themselves what their own values are, you know? Is it more important to have your child own the exact number of Lego sets that you find morally acceptable, or to have him grow into a person whose mother modeled her values but demonstrated flexibility and showed more interest in his passions and less judgement?
I've just realized that one reason that we didn't struggle with consumerism issues with Lego is that my kids inherited a huge tub of Lego from their cousins. New sets came into the house on birthdays and Christmas, but there wasn't a constant hunger for new Lego - they had such a large supply to build their own creations. You could scour yard sales this time of year for inexpensive Lego and then new sets could become a special occasion treat.