First, consider what you want to do. Then go talk to your ex-boss one on one and face to face, and be honest with him.
Don't think of it so much as loyalty as it is maintaining, fostering, and growing a business relationship. This person thought enough of you that he found you a job before shutting down the company. This tells me that he sees something of value in you. He has the potential to be of value to you, so don't throw that away.
Start the conversation with something like this.
Thanks for taking some time to talk with me. I truly appreciate you
finding me a job before the company was shut down. Also, thank you
for the freelance work. However, between my full time job and the
freelance work, I'm really feeling overwhelmed and exhausted.
What you say next depends upon what you want to do. Do you want to continue doing the freelance work, but perhaps at a different pace or fewer hours spent on the work each week? Do you want to abandon the freelance work altogether, which has the potential of eliminating any future freelance work.
One of the unknowns, or at least not stated in your post, is how long do you expect the freelance work to continue. A few more weeks, a few more months, a few more years, something else. If you don't know, ask. Maybe there is light at the end of the tunnel, and the work he needs you to do will be ending soon.