If he was only neutered just before christmas he may still have hormones circulating that are responsible for the spraying. So it may calm down in a few weeks time (I think it can take up to 6 weeks for the hormones to settle down?).
If there are limited areas, blocking them off can help. When Jaffa was spraying in the hall I cleaned it thoroughly but was never 100% sure I got rid of all the smell (it was fine to my nose but I'm not sure about Jaffa's!). Once I got him retrained into using a 2nd litter tray I blocked the area in the hall that he was using with a cardboard box so that he couldn't walk up to that spot and spray out of habit. Similarly in the bedroom - I have had problems with him spraying the wardrobe but I have now blocked off access to that for the same reasons as I think part of that was habit - he walked into the gap between my computer desk and the wardrobe, turned around and then sprayed out of habit. He now can't do that and thankfully hasn't chosen an alternative site.