How do i clean/disinfect a mattress?

A carpet steam cleaner should do the trick. A spring mattress is all air, there's only the ticking material on the top and bottom and the vacuum steam cleaner should be able to reach that. You can also get a water barrier for the mattress and then put mattress pads on top of that and then sheets. You need to have some absorbent padding that can be washed cause the body sweats up to a litre of fluid each night.

If you have a foam or latex mattress then replace the mattress.

Get an enzymatic stain remover that works on urine and vomit. Spritz it on and then use an old clean towel to absorb the moisture. After you treat the mattress and it's dried again, see if it needs a little more enzymatic cleaning. You might have to repeat this a couple of times over several days time until the mattress doesn't smell anymore.

Do get a vinyl/plastic mattress protector to use underneath the regular mattress pad until he reaches school age.

Don't try to mask the smell with things like Febreeze - you have to neutralize the urine with a cleaner.

Use a damp cloth to scrub it, then hit it with some Lysol. maybe some febreeze