So -- a YMCA membership for me and my two kids was about $69/mo -- this provided a child care service -- what would be cheaper - what you want or a gym membership? Sell him on how you are saving him money!!

Also, I would recommend not buying the cheapest one you can find, try to find a better one maybe?
It's not all about the money -- a happy, healthy mommy is a better mommy!!
However, my husband is kind of the same, but partially because he worries about finances, since I only work part-time so I can stay home with the kids.
Space wise, if your parents don't mind it coming in the house, no argument there.
Since he has his weekly "treats", can you put away a bit each week, a little less painful to the budget. I know I have on occasion kept track of every penny I spent in a week and found a lot of waste that could be put away.
Good luck!! I know you can do it -- it took a little effort, but I got my treadmill the same way!!!