What does he mean by "slow down"?

So I've been seeing this guy for about 3 weeks now. We talk all the time, go on dates, have sex. He's met my friends and I've met his. He's told me he likes me and we both established from the beginning that we want something non casual. Yesterday we were hanging out and he freaked out and was like "I don't know if I want a relationship right now" and blah blah. So I told him we would need to end it because I wasn't looking for anything casual. He then started telling me that he liked me so much and that he actually wanted to see where this went but that we needed to slow things down. I'm so confused because he literally acted so into me until this exact moment last night?
What does he mean by "slow down"?
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