Checking a Temp

My baby boy is 6 weeks old and I’m wondering what the most accurate way to take a temperature is? We got one of those head ones that you don’t touch to their head just hold it there and it shines a light and reads it. I was testing it out and I think it gave an inaccurate reading and I tested his rectally and with another thermometer and they both were normal at 98.8 he seems to be acting fine. Eating normal maybe a little extra hungry and sleeping a lot but not really more than normal. Any advice would be great!