hi. are you nursing him to sleep or before sleep regularly? this could be a cause of trouble getting him to go down. also, what is his A time right now? if he's not quite tired enough it may be another reason he's fighting it. at 6 mo A time should be about 2 hours 15 min to 2.5 hours if he takes long naps.
as for winddown, espeically with an older boy running around it may be good to go into his room and let him explore his room in a quiet darker environment for 10 min or so before you start the winddown just to get away from excitement elsewhere in the house. then songs, stories, cuddle on your lap, some rocking etc before you put him down. having a lovey may help. is he swaddled? paci? i never did longer than 5 min at the winddown but often had quieter activities before going into her room (like watching a calming baby einstein video), so i guess this was also winddown.