I'd say the main things are keeping his routine as intact as possible (feeding and sleep times especially), packing extra nappies and wipes than you think you'll need because Bintan can be pricey, and bringing a portable bassinet or travel cot so he has his own familiar sleep space. At 3 months he's still quite portable lah, so honestly it should be manageable - my boy was similar age when we went to Batam and the key was not stressing too much and just going with the flow. Just make sure you have his health booklet and any medical documents handy, and I'd also bring a basic first aid kit since you're across the water. Have a great trip with the in-laws!