How to set boundaries in a relationship?

I think my boyfriend takes me for granted and it's my fault. When he needs something, he calls me and I help him immediately. Actually he calls me almost only when he needs something or is bored. I would like him to respect me and my busy schedule. But I am afraid that if I don't respond quickly enough, he will just find someone else and we will lose this connection.
Any suggestions how to be more connected and show him that I have boundaries as well?
How to set boundaries in a relationship?
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