It sounds like he has an upper respiratory infection which is fairly common in cats and kittens. However, he will need an antibiotic in order to treat it. There is really no way around this other than having him seen by a vet.
While Benadryl can be given to cats in some situations you need to make sure something more serious (like a respiratory infection) isn't going on. Otherwise, you will just be masking the symptoms and not treating them. Even if this was a circumstance in which an over the counter medication could be beneficial dosages are based on weight, not age. Therefore, no one online can properly advise you of how much to give him.
So, call your vet in the AM and get him in as soon as possible. The appointment would also be a good time to set up a deworming/vaccinating/neutering schedule for him.
I agree with EB24. One of the most serious complications with an URI is that the cat can quit eating: if a cat can't smell (his food), he won't eat, which can lead to weight loss and fatty liver disease, especially with a cat this young. Kittens can go down hill fast and die.
This is an un-vaccinated 6 week (maybe) old kitten - you're playing with fire if you try to treat this yourself.
Something else: kittens can go downhill in a heartbeat - and, they cannot go without food for long (cats, unlike us, cannot 'live off' their body fat).
Please - get the kitten to a competent feline Vet as quickly as possible. I'd suggest you look for a cat-only clinic. You can look around or look here (if you're in North America) http://www.catvets.com/cfpandpractitionersearch/
In the INTERIM, there are a few things you can do to help him deal with the symptoms. Here's a reliable** site about cat flu and treatments http://harpsie.com/cat_flu.htm
** "Somewhere" you found the suggestion about Benadryl.....many people nowadays are Google-ing to diagnose and treat their cats themselves. Problem is, Google doesn't discriminate between reliable, science-based vet info sites and those sites with low-grade, half-truthed, or even false and misleading info. The "e-how"s should really be read: e-howNOTs to treat cats.
At the very least, drop by herefirst in future!
As a BTW.......there are some serious considerations for vaccinating him...including not just trusting a Vet to do the right thing. Here's a reputable Vet's recommendation about vaccinations http://catinfo.org/?link=vaccines (NO "adjuvanted" vaccines!)