Can somebody please help! Urgent?

Me and my boyfriend came back from Greece yesterday but the night before he got bit by a mosquito and this morning he woke up with a rash around the bite on his knee but he put Betnovate RD cream on and that cleared it up around his knee but in the last hour and a half the rash, tiny little bumps that aren't any different colour to his skin, have started to appear all over his body, up his back, on hid arms, legs and all over his chest and stomach. In the past 20 minutes they have started to go a little towards the red side. He does have a doctors appointment made for later on but I was wondering whether anybody could help me at all. We have searched all over the internet and just found Scabies or Hives or maybe Shingles but they don't look severe enough and all thatwe'vee read, have said that they appear over days but this has happened in a matter of hours so if anybody could help, that would be great,thank youu.