What if Cersei and Robert had a trueborn son after Joffrey?

[Angel Eyes]

Would he be seen as the true heir? Would Stannis and Renly back him? Or would he be seen, if he decided to press his claim, as trying to leapfrog the line of succession, as Renly did?

[GallowsKnight]

Would he be seen as the true heir? 

Nah probably if anything he makes Joffrey look more legit. A mix of children looking like mum and dad more than each other is not unusual.  (Example the Stark kids all look like Catelyn except for Arya who looks like Ned)

Though if he was very much like Robert people might back him in a plot over Joffrey and a rumour (actually true) might be made up. Kind of the like Daeron the Good rumour. Especially if Joffrey is a little shit

Would Stannis and Renly back him? 

Renly will do whatever furthers he and the Tyrells agenda. Stannis will whoever he believes in the true heir. So if Stannis suspects Joffrey is false he will back the hypothetical kid. If he has no reason to suspect he would back Joffrey even over the hypothetical kid (ironic I guess)

Or would he be seen, if he decided to press his claim, as trying to leapfrog the line of succession, as Renly did?

True or not. There'd be some who would level that accusation at him. His argument would be more legit than Renly as Renly was bypassing Stannis/Shireen as well and didn't know they were bastards until later.