Welcome to Just Answer and thanks for your question. It would be a lot faster (and a whole lot cheaper) to take him to an urgent care center or retail clinic for a quick strep test and some antibiotics. It can take the rest of the night to be seen at the ER and given a handful of penicillin.
If he isn't in bad pain, he should be fine if he sees the doctor Monday morning. It is untreated strep that can pose a danger of glomerulonephritis and heart damage in children. If he's terribly uncomfortable, then you'll have to brave Saturday night at the ER.