Breastfeeding and baby is having tummy trouble

Angelea
My little guy is now 7 weeks old and for the past week he only poops about every other day. I am soley breastfeeding. He goes pee just fine and eats great but I can't figure out what's wrong. Any ideas? Solutions? He has an appt Monday but I'm hoping y'all have some advice before then. 
Update:Went to the doctor yesterday and she said exactly what y'all did. He is growing perfectly and is healthy! At 7 weeks he is 10lbs and 22in!! At both he was 6lbs11oz and 20in! Thank you again for your help and advice!!