Dating

When you ask a guy you're seing if he's free this weekend, and he says "I'll let you know" , does that necessarily mean you're not a priority?

I asked the guy I'm seeing if he's free this weekend. He said ,"I'll let you know" Today is Wednesday by the way. How can he not know yet when he'll be free? Should I consider this to be a good possibility that he's not treating me like a priority? And how can I address it? Even if he has plans with friends, shouldn't he invite me with them? Additional details: We just attended a social event tonight and he wasn't able to do our usual mid-week sleepover afterward because he's taking his mother to a doctor's appointment first thing in the morning. He seemed sincere when he explained that he's taking her because she speaks litle English . I slept over Monday. He also didn't want to see me all last weekend because we slep over Wed and Thur night in a row and he needed a break (without saying it. That's his pattern: needing time apart for 3 days after we have a long date). But does it seem suspect that he "will let me know" when he's free this weekend? I thought that was something people only say when they think something better will come up. Additional details: He told me earlier this week that he considers us to be in a relationship. That was only because I asked how he sees us. We've been together for 4 months. He also kept bringing up how it he feels like he's coming down with a cold.
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On Friday , I asked him if he wanted to see me sometime this weekend or if he wanted to wait till sometime next week. He asked to see me that same night (friday) I guess he meant,"I'll let you know if you ask me again."
When you ask a guy you're seing if he's free this weekend, and he says "I'll let you know" , does that necessarily mean you're not a priority?
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