Ok, I must have read too much into "What I think he fears is a lower search ranking."
So it's not that he fears a lower ranking than what he already has, it is that he fears a lower ranking than what he could have with another platform.

I guess my answer to that is it all depends on who is doing the SEO, and especially how much the person who is updating the site knows about SEO.  I'd agree that it's probably easier for most SEO experts to use a LAMP platform, and there are probably a lot more of them.  But as mentioned earlier, the most important part of getting traffic is getting the actual content on your site that people want to see. After the content is there, people will link to it, and if lots of relevant sites link to the content then it will be much better than having a keyword stuffed URL alone.

Bottom line is that it's not just about what platform has "better" automatic ways of doing SEO, it's more about learning how to produce content that appeals to the people in your target market, and at the same time realizing how that content will be indexed.

I don't think that it's fair to say that DNN is better because of it CMS ability either. There are plenty of apps on the LAMP platform that do content management as well or better than DNN.