I would agree with you that it's likely to be an allergy to strawberries, considering the fact that he just had some, and that strawberries is one of those things that babies are more likely to react to. They sometimes react even when they're breastfeeding and the mother eats strawberries.
If you end up contacting your doctor, be sure to mention the strawberries so that he has that in mind as he makes his decisions. :-)
I highly recommend Desitin--the original recipe, not the more recent easy-to-wipe-off version. I consistently had wonderful results when I used that with my babies. (I'm not quite sure why some people feel a need to wipe it off. I just left it on and let it wear off on its own, and added more layers as I needed to.)
I also recommend leaving the diaper off for as much time as you can.
I'm not sure if petroleum jelly (Vaseline) has mineral oil in it, but I would recommend not using any rash cream that has petroleum jelly in it if it does. Mineral oil, commonly used in mainstream "baby oil" brands, leaches fat-soluble vitamins out of babies' bodies and thus is not good for babies.
I agree with those who advised you to take care to not use the cornstarch if there's any chance of a yeast component to this--in those situations the rash does get worse.
Here's some more info....
http://www.askdrsears.com/html/8/t081400.asp
http://search.llli.org/site/apps/s/google_search.asp?q=di...