I’m no expert as you may have read my posts but just a suggestion. If he won’t nap when his brother is home, or he has a really short nap, could you do an earlier bedtime?
When my LG was around this age I would offer an earlier bedtime, at least 30-60 mins earlier, if her nap was 45 mins or less. As she’s gotten a little older she was able to come better on a shorter nap and we then needed to cap at 45 mins to preserve bedtime. She’s recently dropped it altogether and if she has even 30 mins, her bedtime is mega late. She’s now 33 months old, so just shows how things can change so quickly x