Batman thinks he generously let Robin free to explore the “wild blue yonder” (line 3). Robin's view of events is more down to earth - “ditched me ... in the gutter” (lines 4 and 5). We wonder what really happened between them.
Robin uses various clichéd phrases as he tells us that he has “scotched that ... rumour, sacked it, blown the cover... let the cat out” (lines 6-9). The casual language makes it sound as if he is showing off to his mates, boasting of the power that releasing these secrets has given him over Batman.
The casual use of “motor” for the Batmobile also suggests he is trying to impress us.
Lines 12 and 13 are spoofs of tabloid headlines. "Holy" is an adjective that Robin used a lot when exclaiming, such as “Holy smoke!” Here, Armitage has exaggerated it for comic effect. The 'new' Robin seems to be making fun of his 'old' self.
Robin becomes scornful when he uses the metaphor "I'm not playing ball boy any longer" (line 14). He's fed up of the 'game' and doesn't want to serve anyone any more.
He makes fun of his superhero outfit, again sounding scornful by describing it as a fashion writer might - “that off-the-shoulder Sherwood-Forest-green and scarlet number” (lines 15-16). He prefers “jeans and crew-neck jumper” now that he is an ordinary kid.
It is ironic that he calls Batman “baby” in the final line. Although it is a term of endearment, he uses it to belittle Batman.
Robin the kid has grown up, while Batman the superhero has diminished to a baby.
The phrase “boy wonder” has a new meaning because Robin has managed to break free of his idol.