Any ideas what this could be?

It’s on my toddlers arm and is the only one on his body. It’s about a cm in diameter- dark pink around the outside, then red inside with a white (possibly pus filled) head in the very centre. I squeezed it gently and it feels like the spot has a hard “base”, same size as the redness on the skin.

To me it looks like a whitehead spot that’s become inflamed, but he stayed with my parents over the weekend so I asked my mother if she’d noticed it and she’s now panicking because it’s gotten worse overnight and she’s asking me to take him to a&zero incase he gets blood poisoning and obviously it’s making me worried.

Any ideas what it could be? I was going to put an antiseptic cream on for the night and then take him to the pharmacy tomorrow so they can advise me but I’d never forgive myself if it is something I should’ve taken more seriously 😑