Hi
I just use the nozzle attachment on our hoover to hoover the mattress (seems stronger than when I use the upholstery attachment) and I think it just helps when changing sheets etc. I wash duvets and pillows six monthly - but the pillows tend not to last really so usually get replaced in January sales or at TKMax ;-) about once a year. I think this is a habit I've just picked up from my mum though as in theory if the pillows and duvet are encased in dustmite proof covers all the time they shouldn't need this (whereas the bottom of mattresses is usually exposed..) - though have never checked advice about this tbh.
I'd give it a while with snotty tissues to see if it eases - not least because it's winter so may be to do with that. I think nose spray helps me temporarily if nose feels blocked, but doesn't help itching/soreness or get rid of problem. Also, bedroom is important but unfortunately it's the whole house that's a problem - is there anywhere else in the house (living room etc) that he spends a lot of time/close proximity to soft furnishings etc that may need addressing?
I was also thinking last night if you've had a diagnosis for dustmite allergy? If not, it may be worth asking for tests as there may be other triggers you aren't aware of? Also - and this is probably very obvious - but any pets? People always think of cats/dogs, but actually for us its hamsters/guinea pigs/rabbits that are a problem because of their bedding.
If the roller blinds in his room are material I'd hoover those! (Also light shade?). If vinyl/plastic/wood I'd wet 'dust' them.
Over the years I've tried so many different remedies (as have my siblings) and none of us have found anything that's made any significant difference tbh. We have often thought something was helping, but then realised that overall it hasnt really - just us being optimistic! But I'd really like to know if the Nigella seed oil does help as haven't tried that one. I and my daughter have to take anti-histamines when we're going to stay anywhere which she hates (I'm used to it) but a more natural alternative would be great!
Other thing that occurred to me is about your son's clothing. I wonder if it's worth washing and tumble drying all his clothes and keeping the clean ones separate from the ones that you haven't done yet to stop any mites transferring in wardrobe/drawers. The backs/bottoms of wardrobes (inside) and drawers (inside, usually back corners) are real dust gatherers that because they are inside people often don't think about. It may be worth taking everything out, wet dusting, and replacing washed and dried clothes. I wet dust our children's wardrobes and drawers each month (when I do mattresses/curtains) to keep on top of it - I just take everything out, wipe round and put things back in - but if it's the first time you've done it, it may be worth cleaning all the clothes too. Big job initially though...I'm not so stringent with my own wardrobe/drawers I must admit, but still do this a few times a year - and I do start to react to the dust when it needs doing!
I'll watch the thread to see if you update in a while about whether things ease for your son, and whether the nigella works!