It's not a complicated question though, as it just requires a yes or no answer.
Just because the answer can be a simple yes/no, it doesn't mean the decision is necessarily a simple one. He may need to take time to consider the options, or he may need to check with someone else.
He read my message on the chat program we use, but he did not make a reply. I sent a follow-up message a few days later, but I'm worried about what to do if he doesn't respond to this follow-up message either. We all work from home, so it's impossible for me to talk with him in person.
As your line manager his job is to guide your work. He's not doing that if you have no regular lines of communication. Your boss should be managing those lines of communication, but if he's not you should suggest to him what form this could take.
My team have a daily "Standup" meeting early in the day where we give a quick summary of what we've done, what we're planning to do and anything that is blocking us. This is standard practise in our industry (software development). The meeting is quick, so you don't necessarily do into the details of the blockers, but you do agree what the next step to unblock the problem is and who will take that action. Because the meetings are daily there is a regular opportunity to update on progress being made or deadlines getting tighter.
Another common practise is to have a scheduled 1:1 meeting with your manager, maybe once per week. Working remotely that would take the form of a voice or video call. Because it's a conversation between two people your manager has to answer the questions you ask them. It might not be the immediate yes/no answer you're hoping for, but they can tell you what their next step will be (e.g. checking with someone else, or even just spending some time thinking about it). If that's going to cause problems you've got an immediate opportunity to let them know - e.g. if there's a deadline that has to be met.
Another answer mentioned that managers are extremely busy. That's often true, but as with any role they need to balance their responsibilities and let their manager know if they have too much work for one person to manage. One of their responsibilities is to line manage you, and they need to find time to do that. Middle managers often have packed calendars, so you can help them to dedicate time to you by making sure that you've got a scheduled slot in that calendar.
