You can at any time developed an allergy to it. My youngest brother was highly allergic after his fourth/fifth? sting and would require an immediate visit to the ER every time he got stung after that, but he's not been stung since his teenage years (he's 30y now) so we don't know how his body will react to being stung now. My younger brother (just turned 32) just a couple summers ago got stung for the first time since childhood and had to make a run for the ER because of developing breathing problems just an hour later.
I'm scared to get stung myself now cause both my brothers are now walking with epipens and the last time i got stung at 17y I ended up having to take steriods to get the swelling to go away from two hornet stings I got at a pool party.
However, my daughter has been stung countless times (12y) and just this summer stepped on a bee and her foot swelled up and was swollen for two days- not a usual reaction for her! So she might be starting down the path to being allergic....
Sooo, if I were you, I'd have the pediatrician check him out, but I don't think he will give you an epipen just yet. Just recommendation for benedryl and ice packing it. If anything, having the facts of his reactions to the current sting might be useful in the future in case he gets stung again.