We just received our immigrant visa this month. We are planning to migrate during the 1st week of August, but my son's passport will be expiring on February 23rd, 2016. Can he still use this passport? We are planning to renew his passport at the Philippine embassy in the US.
Most immigrants are required to have a passport that is valid at least 60 days beyond the expiration date of the immigrant visa. There are some exceptions.
These are described in section 211 of title 8 of the code if federal regulations: https://www.law.cornell.edu/cfr/text/8/211.2
When we moved to the US recently, we were told by the local US embassy that it was best to have passports valid for at least 6 months after the expected date of travel.
This was not a requirement to allow us to enter the US. It was because, supposedly, airlines can refuse passage for travelers who are traveling to a country that they do not current possess citizenship or residency, and do not have at least 6 months of validity on their passport. (Remember, an immigrant visa to the US is not residency in the US. You only get a green card after you enter.)
In your case, it seems that you'll have over 6 months of passport validity, so there should be no problem at all. Your son's immigrant visa is only necessary to enter the country. Once inside the country, his documentation to remain will be his green card, so once he receives that, his Philippines passport won't be necessary for residency purposes.
So if you do indeed move in August, your plan sounds perfect. Just travel on his existing passport, and wait to renew the passport in the US.
If the move gets delayed a month or more, I suggest you ask your local embassy for advice. They were very helpful for us. In a case like that, you might want to renew his passport in the Philippines, and bring both passports (the expired one with the immigrant visa, and the renewed one) on the flight.