10 week old son started the day with normal feeds but then started sleeping a lot and not really eating which is opposite to normal no temp any idea?

Sleep cycle change?: You still need to wake him up every 4 hours to feed him. Also look for the signs of illness -fever, lethargy, vomiting or fussiness or dehydration - less wet diaper than usual. If you see these signs , he needs to see doctor in the clinic.

Poisoning?: Any young siblings in the house? Sometimes they want to be helpful and may give the baby something that they should not, like a medication, food or other item(s). Also (I have seen this personally) parents have mixed up formula by accident with an alcoholic beverage. Have child evaluated immediately.