The Contented Little Baby Book by Gina Ford - amazing!
Their circadian rhythm isn't fully developed until they are 6 months. Some babies get there quicker than that. The only advice i could give is to make sure daytime naps are different from nighttime ones. Like daytime is in light with music or household noises, nighttime is dark and more relaxed. I understand you say he can't be kept awake but still try to wake him after 2-2.5 hours even if just for 30-40 mins
My little girl is 2 months and things I’ve always done and she seems to know the difference is at night she will have her bedtime bottle upstairs in a dark room with white noise on and just a dimmed red light. I put her in her sleeping bag and she seems to know that’s when it’s bed time, I don’t speak or anything either as she will then think it’s time for her to be awake. We get up at 5:30 every morning, she will have her bottle downstairs and then I will interact with her. She does sleep long hours throughout the day but only up until about 4/5pm then i get her bathed and into her pjs and she just automatically knows it’s nearing her bedtime as she just won’t settle downstairs till I take her up. Every baby is different though, I introduced routine very early on because I just can’t function without one 🤣 it’s all trial and error, babies will just fight sleep xx