Wow, you sound pretty angry.
Take a deep breath and remind yourself of why you're not with him, probably it's because of situations like this.
You are right in that it is his weekend and he should take responsibilty for his son as agreed.
I think you did right to make a stand. A child is a responsibility and he needs to take that responsibility. He can't just skip a visit because he's too busy, being a parent doesn't work like that. He made a comitment to once a fortnight, and he knew well in advance which weekend it was going to be.
However there are sometimes going to be times when for whatever reason he might not be able to meet that commitment. And there are going to be times when you want to change the weekends to suit you too. So a certain amount of flexibility is always wise. And he could of course have just made arrangements for someone else to take care of your child while he went to work without even telling you he'd be working...
If it were me i'd probably prefer for my ex to ask me first if i would have my child instead if he was unable to make his commitment before he started asking round him family, friends, neighbout, anyone off teh street who'd say yes... Can you honestly say you have never asked a friend, neighbour, family member to have your child because you need to do something? Whether it be work, doctors appointment, shopping...
Of course what he actually did was ask you to ask your own relatives rather than his, or even him asking your relatives. So you need to make it clear, if he has a commitment he can't get out of (and it better be a bloody good one) which means he can't see his child, then probably asking you if you will swap to another weekend is best. Then if you can't swap as you have made plans then it is his responsibility to arrange for someone to take care of him.
I would also suggest that, excepting emergencies, any changes to agreed contact time should be requested and confirmed in writing at least 6 weeks in advance, by either of you. Then you both know where you stand and it gives you plenty of time to inform your son of alterations so he is not disappointed or let down at the last minute.
Hope you enjoy your shopping
take care
Gilly