Mattress help!

Hello all, my little one prefers a firm surface to a soft one. And when I say firm, I'm talking rock hard. He's 8 months and we have a standard crib mattress but he's been having restless nights. We've tried all the tricks in the book it seems like and nothing has worked. It wasn't until now, at 11pm that I was thinking...maybe it's the mattress. He sleeps perfectly fine in his pack n play, or on his foam puzzle pieces mat. We also visited my grandma recently and she has a mattress that literally felt like laying on a freshly paved road 😂 my little man got fussy so I brought him into bed with me and he was out like a light.

So...does anyone have suggestions on an extra firm baby mattress? The only reason we don't have him sleep in his pack n play at night is because his nursery, and our room are upstairs and his pack n play is downstairs with the attached changing table. So it's convenient to keep it there instead of bringing him up every time he needs his diaper changed. And of course, we take it with us when needed so I would rather just have both.