Based on what I've read online and videos I watched. I'm not a physician.
Is hooking up with someone who had Covid in the past a lesser risk? - Yes, but he can get infected again and infect others. There's evidence that even if you had it, you can get it again. Also, I assume he can also pass it if he has it on his clothes, sheets, bed, etc.
Wouldn't he have the antibodies? Yes, but they are not sure how long do they last. Some say about 3 months. I haven't read conclusive evidence or a consensus on this. And it seems like the virus can mutate too.
I really don't know how it works. No one knows everything about this virus. It's a new virus. There's lots of research going on on this. Information and recommendations are changing constantly.
I know i'm 100% negative. You cannot know that. No medical test is 100% accurate for many reasons, including error and timing of the test. Because it is possible to get a negative result even when you have coronavirus, it is important to be careful even when you receive a negative result. Some doctors recommend quarantining yourself until symptom-free for at least 72 hours. More about tests https://www.fda.gov/consumers/consumer-updates/coronavirus-testing-basics
I haven't hooked up with anyone since February. Good for you
I'm getting tempted yet still wanting to keep myself safe. You are doing the right thing. And we all take the risks based on our own circumstances. Some might risk it and it's fine if they are aware of all the consequences.