Lump under chin :/

Hi

So this is Ziggy. Yesterday I went out and we had a heatwave so he did at one point in the day his tank hit 22 (I live in England this never happens!!!) I lowered the temp when I got home he will have been that hot for a few hours max. He is now at about 18 degrees

So when I woke up this morning he had a small fugus infection on one of his gills and his jaw looked a little larger than usual but honestly it was such a small diffrence I didn't think much of his jaw.
Because he was still eating and his usual self and tomorrow is his waterchange day ( and I was at work) I thought a he could wait till tomorrow for his usual water change and I could give him a salt bath then.

I have just gone to see him now and his chin has what looks like a red/purple absess on it which was not there this morning.I'm quite worried about what to do now.
He is still acting his usual and he is eating fine.

Photos of his chin and his gill are attached

Water is (tested with liquid tests)
Ammonia 0
Nitrite 0.25
Nitrate 20-40 (inbetween the two colours on the test so im guessing about 30)
Ph 7.2

Any ideas on what to do with him :/

Ok. So many replies. Thanks

Update incase anyone's reading this I gave him a salt bath and did a large water change. The fungus has gone and his chin is smaller than it was. Not 100% yet and he did have some trouble eating for a little bit (still had a massive appetite though!) But fingers crossed he is on the mend