you've been waiting all week for him to text. now he does and offers you two options for the weekend. why on earth would you pretend to have plans?
how utterly childish and absurd.
If you want to go out with him, go. Your Mum offered to babysit and you haven't made plans yet, so make plans with him if you fancy it.
Jesus, everything will be hard if you're going to become that complicated.
If he is used to you making all the arrangements it probably hasn't struck him until today. I agree with Millie99, my DH is also very disorganised about social plans and this is exactly the kind of thing he would do.
How is it rude to make plans with someone near the time you'll be free? I find the closer I am to people the more likely I am to see them at short notice. Long term weeks in advance plans are often with friends I see less often and am less close to. My best friends are the ones I call on Friday evening just to see what's happening and hook up with if it suits.
Sounds to me that he assumed you guys would hang out this weekend as that's the pattern, and when you didn't arrange it he wanted to see what was happening.
I would say you provisionally have your answer to the "is he interested?" question. The next question is for you: are you happy to go out with a man with whom either you make the plans or things happen last minute? [i can think of worse things]
On the other hand, if the reservations you mention are strong and he is your boss, then perhaps you should back off gently.
Please, whatever you do, don't make some stupid "principled" stand over the timing of his text. You wanted to see if he'd get in touch. He did. Now respond in an honest, mature and respectful way.