I can’t sleep on my boyfriend’s mattress

My boyfriend has a hand me down tempurpedic mattress which he loves and when I don’t spend the night he sleeps in the middle of it. Every time that I lay in it with him I sink into his body and we don’t even have a crazy weight difference (~60 lbs). This never happens on my bed, nor others from hotels, etc. I tend to get spasms after sleeping there because I think I overcompensate and twist my body to avoid falling into him. He doesn’t like to spend the night at my place because it’s “out of the way” and I want to be able to spend nights together. Is there any way to prevent my body from sinking into his on the mattress?