One more thought -- I still think the BT resistance might be OT. Is DH around at that time? Next time, try parent switch. When DD is very OT at BT, it takes ages for me to settle her - she will keep crying or settle-WU-settle-WU -- then we switch and she snaps out of it. It works the other way around also when DH can't settle her.
The reason I am saying this is, DD and L seem similar in their sleep needs and I have noticed this happening with her. Day Before with A of 3.50 -- 1.20 am nap followed by 30 minutes pm nap ending at 4.30. I aimed for BT at 6.30 - 2 hours seemed fine. But nope-- very OT and needed help. Yesterday am nap of 3 hours, 30 min CN ending at 6, BT at 7.30 - slept off so happily. Today again, WU - 7.30, am nap of 1.5, CN 30 mins ending at 5, BT 6.30 - no issues (yet)
So, yes, completely agree with Marta, that the resistance is probably due to him already being over tired by that point rather than UT. You mention that after capping the nap he would WU soon after BT, that usually means OT.
You can of course push BT to 7.30. In fact Tracy even recommends a 7.30 - 7.30 sleep cycle (I think) Totally agree with Marta about the 'end of the day'. In fact, I remember talking about this in the BC. That as long as she is rested enough, she will switch off at that time because it isn't just the A before bed but rather accumulated tiredness of the day.
FX that the good nights continue!