Hi sorry about your dog having problems, mine is also right now (see above) Neosporin is supposed to be good for that and check the "Merry Clinic" google it if you want to go the natural route. Good Luck, boxers are so cute=)
First of all, it's best to take him to the vet to be absolutely certain of what you are dealing with before you treat him for anything. If it's not psoriasis but something else instead, then the things that might possibly help psoriasis might aggravate another condition and make it worse.
Is there any way you can post a photo closeup of his face? I am suspecting that it is probably a case of either puppy acne or plastic dish dermatitis rather than psoriasis.
Puppy acne usually crops up as the dog begins to approach puberty, usually between the ages of 5 and 9 months. If he's 18 weeks old, then he's one week shy of 5 months and puppy acne is a strong likelihood. It's extremely common in boxers, and since he is part boxer, the odds are in his favor that that's what it is. There is really no treatment for this, if it's puppy acne it will self-correct after he passes his first birthday.
Do you feed from plastic dishes or metal ones? If you use plastic, he could have plastic dish dermatitis. This is common in all breeds and it happens because the plastic that the dishes are made from contains an antioxidant that many dogs have a reaction to. Plastic dishes are also one of the possible causes (along with hormonal and genetic) for the puppy acne I mentioned above.
Neither puppy acne nor plastic dish dermatitis have any real treatment other than switching from plastic bowls to metal ones. If you switch the dishes and the condition does not improve markedly within a short period of time,then please consult your veterinarian before attempting to treat him on your own. It could be that it's a more serious condition that needs actual medical attention.
Please let us know what happens.
Ghilly