We are about to take our 18mo camping and I’m wondering what the safest bedding option is now that he’s out-grown the portacot? Thanks

As your son is beyond age where SIDS is a risk, the main concern is that of safety.
Portacot would generally be recommended for an 18mth old, depending on weight recommendations.

Red Nose info re:portable cot.

Do not use a portable cot if your child weighs more than 15kg (or check instructions of your particular model).
https://rednose.org.au/article/portable-cots

As this is an issue you could use whatever bedding you are using for yourselves….. ? mattress/airbed, but remember he will no longer be ” contained”