My son is 10 weeks old and he has the same problem all along - he can sleep as long as 4 hours in the afternoon.
And...then I keep using wet cotton pads to wipe his face, especially his nose / ears to wake him up as babies is very sensitive to water... and then change nappy ... keep talking to him.
If he sleeps more than 2 hours in the afternoon, I'll wake him up by changing his nappy and wiping his face. Sometimes, he will cry and fuss, but the fussying and crying will disappear in a week / two weeks.
Have to keep this cycle feed / wake / sleep ... then babies can sleep much better at night.
my baby also sleeps a lot a lot when he was two months old. when he steps into nineth to tenth week, it will be better. He used to sleep about 18 hours a day!