Why is this? Everything I give him, he tells me it's boring.

if his happiness is full, you can't give him toys to play with. having a minion keeps a pet's happiness at its maximum, so if you want to be able to play with your pet you will have to remove the minion and wait for his happiness to deplete. although, there was an issue recently with happiness levels getting 'stuck' regardless of whether or not a pet had a minion.

thanks! I'll keep an eye out if mine gets stuck.

Just a note, but if your pet has a minion, the minion keeps the pet at permanent full happiness.