wood in cage

To answer the question if the wood is safe to chew, you need to tell us exactly what kind of wood it is and whether it was treated or not.

Sandpaper is a very bad idea. It can really hurt your piggie's little feet. Would you like to walk on sandpaper, i bet not?

You could cover the ramp and landing with carpet pieces, shelf liners, or fleece to make it less slippery.

Also, it is normal for you pig to be hesitant about the ramp, you may be able to lure him up with veggie treats after you covered the ramp with something.

Piggies are not good climbers, like rats or hamsters, you need to make sure that the ramp is covered in something that their little feet can't slip on and that the ramp is not too steep.
If you could post a picture, somebody might be able to give you some more concrete pointers.

I'm not sure if the wood is safe because it depends what kind it is (like Lil' Fella's Mom said).
I've tried a wood and carpet ramp, but it just got really icky from their pee. I wouldn't recommend you making that sort of ramp unless you're positive your pig won't pee on it.

For me, towels have worked best as a ramp covering. Towels stick better than fleece and other materials.

It would be very helpful if you could post a picture of the ramp. We could maybe see where your pig might be having trouble.

We lowered his top level and he now has no problem. It is not pressure treated wood. Just plain old wood. But he loves it and he runs and climbs all over it. He figured out a way to get up there! He does chew on it though. But it isn't pressure treated.