Yes, doesn't sound like there are likely "damages" here, so getting lawyers involved seems unnecessary and counter-productive...but without question you should now publicly demand that he cease and desist from claiming copyright over your intellectual property. You could be 'political' about it and provide him with an 'out' if you so choose...something to the effect of "I'm sure you this was accidental on your part, however, as the copyright holder of this photo I am obligated to request that you remove your 'copyright' claim to this intellectual property."
If you knowing let him claim copyright it is highly unlikely that you could at a future date enforce your copyright. It is entirely feasible that a court could potentially rule against you in court if you choose not to PUBLICLY send notice to him of his copyright violation because you in essence were not attempting to enforce your intellectual property rights.
I'd suggest getting this out in the open. If you want to be nice about it, simply phrase it in such a way as to let him acknowledge his 'oversight' without any real embarrasment... If you want to really call him on it, don't be so nice and make a statement that describes his lack of response to your PMs and re-iterate forcefully that you demand he cease and desist from using your intellectual property without your permission on the forum or anywhere else.
IMHO, say something publicly, either politely or forcefully. He is incredibly out of line to even use someone else's intellectual property let alone claim it is his own!