justwokeup
I’d add get baby food and your meals prepared if possible in advance. When you’re together try to do no housework or as little as possible. Afterwards tidy then sleep for as long as you need to recover! Find out the key things that are important to his mum and dad, naps, food etc and try to be consistent with them. Try to have your own supplies of baby food, nappies, cups, plates, sun cream etc so it doesn’t matter if you don’t have access to theirs at any time. A second pushchair is useful, it doesn’t have to be new but make sure it’s easy to use one handed and lightweight. If you can get any help do, eg a cleaner for a couple of hours, ready meals in stock for after you’ve had him, library story groups, toddler groups etc give you a break too. Above all, don’t have any preconceived ideas about how you expect him to behave and accept him just as he is - we had to tailor our ideas so much to the wonderful little individual that is our gc and every childcare day is full of surprises and challenges. Wouldn’t have it any other way.
Excellent advice justwokeup
Assuming little one's at your house, 2nd hand (sorry preloved), high chair, buggy, travel cot (new mattress obviously) - they're invaluable. Switching car seats is a pain too. (New or one where you know its provenance)
Re buggy - check it's easy to push for your height, lightweight (as justwokeup said) and quick and easy for you to fold while carrying baby or holding hand of toddler.