I see. Thank you for clarifying this for me, Jan.
There is no legal document that you can require your boyfriend to sign which would protect your assets typically since he would not have an interest in your assets in any case since you will not be adding him to the title
nor will you be remarrying.
However, your concern is well founded about being able to evict him if you decide he should not live in your home any longer for some reason and he is unwilling to move out voluntarily. In that case, you would normally need a court order evicting him which can be carried out by the local sheriff. There is no way to force him to waive this right typically but you could enter into a simple lease
agreement setting out the fact you do not intend to charge him rent for certain chores he agrees to perform on your property so there is no question that he is a tenant and not something more.
In the agreement, you could also include a provision that he would be liable for attorney's fees and other costs if you are forced to seek an order of eviction after you provide him with notice to move out and he refuses to leave.
Here is a simple lease agreement that can be used to draft your own based on your own needs:
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