Being spoilt is the parent's fault not the child's - they don't spoil themselves!
You don't discipline a child for being spoilt any more than you discipline a child for lacking a skill that you haven't taught them. You stop spoiling them!
He needs less of everything, not as a punishment but because he's being harmed by overwhelming amounts of everything - that's what being spoilt is, it's being harmed.
One sport so he can actually get good at it, not 3.
Cubs or beavers yes fine.
Technology out of his bedroom - into the communal space. All of it, TV especially.
Fewer presents but ones he actually wants - talk to relatives, ask them to just give vouchers or charity donations or money if that suits them better if he gets masses of "stuff" he doesn't want but is expected to "be grateful" for.
Stop with the big days out with complicated unspoken or spoken expectations of pretending to enjoy himself and be grateful - just do much shorter, free or cheap simpler things like walks, library visits, visits to very local cafe for a hot chocolate just for half an hour, and only require his attendance and silence if he's doesn't have anything nice or neutral to say - no trying to make a specific facial expression or fake happiness compulsory, simply not being obstructive on the most basic level.
Remove everything but furniture from his room and ask what he actually wants back in there - not as a punishment but because too much stuff is overwhelming.
Too much of everything creates awful pressure for some people - it's not not just the too much stuff materialism issue but more importantly overstimulation which creates a constant feeling of having demands put on the person - including the demand to be happy and grateful.
Strip it back - he needs less of everything and time to be bored, not as punishment but because too much of every type of everything from every direction all the time is harming him.
Obviously technology in his room is the epitome of this but not the only problem.