Different temps in fever...

My son is 1 years old and has a fever but it’s weird I’m a little confused I always check his temp under both arms one arm it’s 99.1 the other arm is 100.2 why am I getting two different temps also I know checking temp under the arm you have to add a degree so technically his temps are 100.1 and 101.2 correct?? Also at what temp do I take him to the er?