DS was almost 3 when dd arrived, so a bit younger than yours but I worried about some of the same things. Dd is 8m now and they adore each other. Watching them laugh at each other is priceless.
Things that helped:
Loads of prep about babies, explain that all they can do is cry/feed/sleep and it's a bit boring for them and us, it'll be a long time before the baby is any fun. Whereas he can play, run, chat, watch tv etc...much more fun.
Explain about bf and have a basket of special magazines/books/toys that you can do with him only when bf. I was v worried about bf as had spent hours on the sofa with DS, but he took an hour to feed, dd v luckily would feed in under 10 mins, and much more effectively. I've just finished bf at 8m and it's never been an issue for DS.
Spend the last month of your pg doing quiet play rather than "making the most of the time left" to go out and do exciting things. I was forced into this with spd but it helped loads that DS didn't see dd as the reason why all the fun stopped.
It's going to be his job to teach her the fun stuff, like how to smile (DS took this v seriously and was so very proud when she did her first smile!) how to laugh, sit, walk, jump...
Involving him in as much as possible - he helped pick out some soft toys and clothes for her, and once she was here he chose her outfits for the day, fetched nappies, brought me drinks of water. Always got DS to introduce the new baby to visitors, so he could show her off and it meant he got the first attention too.
Giving DS a toy from dd was a great help. Also, when DS comes for the first visit, do NOT be holding your new baby. I got DH to text on arrival so I could put her down, even if screaming and focus on DS, who had clearly missed me.
Plan for the birth, DS was prepped about waking up to find someone else looking after him. Not needed in the end as dd was very obliging and waited for nursery to open for labour to hot up and dd born before lunch, so DS never knew any different. I have been the babysitter for a friend too and her toddler came to stay with her parents in the room she'd be sleeping in a couple of weeks before due date. Really helped her for when she came to stay.
Survival: agree above re cbeebies, disney films, soft play. Let go any guilt now about tv, it's your friend. Just watch with him whilst feeding etc. Do not stop nursery/preschool attendance. Get a cleaner if poss. Take any help offered (and it's offered a lot less second time around) but don't let it always be to take DS away to play. Lower your standards even further than with your first!
The thing we did wrong was underestimate the impact of school. Dd was born in April, DS was great with her if a bit more demanding. He started preschool in sept and this additional upheaval and massive tiredness sent his behaviour off including with dd. Nothing dreadful, but over enthusiastic pulling cuddling and such like, also wanting to be babied including regressing with wanting to be fed. He was clearly upset and insecure. I thought were were home and dry by then but I underestimated the time the adjustment would take and with hindsight I should have been more careful to give him 1to1 attention in the long term, not just for a few months.
Sorry just realised how long this is, will shut up now!!