3 week old baby boy

My son is now 3 weeks old and the first week was great he would sleep for 4 to 5 hours in between feeds at night but now is waking up every hour or 2. any suggestions to get him to sleep longer? I am breast feeding and formula feeding but I only give him one formula at night around 9 before we sleep and breast all through out the day and when he wakes up at night.