Give him some time. Behind the filter is a common place for males to hide...and on the odd occasion, breed
Caves are generally made to have an opening of 2.5-3cms high and 4-5cms wide but most bristlenose males are not picky. as long as they can fit in there and its kinda snug the they are happy
hell, my latest spawn happened between two bit of wood, even though there are two caves in the tank...
tho that male has always been a bit odd!
give him some time annd he will be fine. I bet you good money that at night he is out susing the tank out, checking out the laidies, sharing a meal etc. in time he will decide where he wants to be, and it may be behind the filter. not too much you can do about it. I had a monster male once who would spend all day behind the filter, and all night crusing the tank. he was happy as larry.
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