Line his mouth with something to keep the swab from hitting his actual tissue. It would help if he knew if it would be a cheek swab or a palate swab. A retainer would work on the latter. The former is a bit harder, but not impossible. Think flesh colored cloth. Or bite wings used to hold the teeth in place prior to dental x-rays. Or a cut up dental dam (or condom) carefully placed in the mouth.
Put someone else's saliva into his mouth. The easiest way to do this without asking for help, would be to find someone's abandoned (but recently used) plastic water bottle with a couple sips left in it. There would be plenty of backwash.
Put in the blocking material. Sip the remaining water and hold in the mouth as long as possible. Don't swallow at all. Let excess water drip down the throat or spit it out. It would not be unreasonable for someone to sip water just before getting swabbed, since opening a dry mouth while someone pokes around in there is uncomfortable. He'd have to time it well.
Will this work?
There are 3 basic ways to collect DNA for testing: Saliva (requiring a fair amount; it can be done with far less, but not reliably), blood, and cells from the inside of the mouth, usually the cheek. (There are other methods such as from dead bodies, bone marrow, forensic from hair, dried spit, etc, and from amniotic fluid, but none of these are relevant in this case.)
I think it would be hard to eliminate his own DNA from the sample. But probably easy to contaminate the sample with someone else's.
But given that you're looking for what your character thinks will work (or is his best chance at working), this is it.
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