Do you have his "Death Certificate"? I am assuming not, because you were not his "spouse"... and therefore, I doubt it would be given to you. Then, there is no way to "prove" it, or that he is the Dad of your son.... and YOU are the only parent on his birth certificate.
But you were not married, and are a "single-parent." And what about all those other Moms/women, who have a child by artificial insemination? There is no "Dad" on the Birth Certificate either.
I would, ask an Attorney.
And, call the department that issues the birth certificates and ask them.
You need to call the airlines you booked your trip with. And ask them.
Or, ALSO go to the "TSA" website and per International travel, you have to read the guidelines.
you have to get Passports for yourself and your child. This takes time to process and costs money.
Ultimately, you have to find out and then attain, all the legal authorizations for travel with children, & passports.
Just some things off the top of my head. We have traveled internationally... and these are the things we had to do and consider. It takes a lot of prep... before the trip. Ahead of time.
Passports, if you do not have one, can take time to receive. I don't know that by July, all your documents would be arrived yet in your mail.