So sorry to hear you lost a turtle. I also have a painted but mine is a midland painted. I am surprised but it appears like both of our turtles are around the same size. I thought the Southerns were a tad bit smaller.. Maybe you have a female there.. I do suspect females must grow faster to reach their mature size for mating.
Phil is our first pet turtle so I don't know about the spot. I was thinking though, since you don't know why your other one died, have you changed out the water in this little ones tank? I think I would clean the whole tank out.. Even any ornaments or rocks you have.. Just to cut down on any bacteria or virus the other one might have had.. Just scrub everything off with hot chlorinated water if you have city water... I would just do it as a precaution.
I would love to see a top picture of him. Those patches do appear to be suspicious of a sort. Phil has a different tummy but his yellow or cream part does not have any deeper color or patches. Is his shell soft at all? Has he/she done any shedding yet?
Phils tummy.. I know its different then yours, because of his markings.. but now looking at it, it does have some deeper darker patterning on there.. I haven't noticed it till I looked closely at this pic..lol He is completely dry in this pic. I don't know if they would look darker wet..

Midland Painted--Philbert--SIP

1-2 1/4" Mississippi Map Turtle(Male)--Atlas
1-2 1/2" Western Painted Turtle (male)--Titan
40 gallon tank