Joshua Terrill always wears his Spitfire costume. Always. It's what he was wearing when his father froze him. He wears it consistently after he breaks containment, even when he's powered down.
(The Ray 1994 #18)
Even in flashbacks to his childhood in the 1950s, he's always in costume--we only see him in situations where he is using his powers.
(The Ray 1994 #18, 28)
Does he even have anything else to wear?
Beings of light like Joshua and his father and brother wear light-construct costumes when using their powers because that's the only clothing that won't burn off when they're powered up and it protects the ordinary clothing underneath. Presumably Joshua has civilian attire under the costume (he went out to breakfast with his father before being frozen, and he couldn't have done that in costume in 1954 without attracting attention, which I'd think would be the last thing a man about to cryogenically freeze his child would want).
(But note that father and son are both in costume when Happy freezes Joshua.)
(The Ray 1994 #28)
It would make sense for him to switch to civilian clothing in situations in which he's not using his powers. But still we never see him out of costume, even when it would make sense for him to be (while eating at a diner, while with Ray--who is in civilian clothing--and Nadine at a police station). He even sleeps in his costume, cape and all.
(The Ray 1994 #20, 21, 24, 28)
This says a lot about Joshua's upbringing. He was forced into a hero role at an absurdly young age and has never gotten to be an ordinary child, and the constant presence of the costume is emblematic of that.
It's also interesting that Joshua's costume does not include any kind of mask or other method of concealing his identity. His father's costume does, and when Ray is taught how to make his own costume, his father insists on his wearing a finned helmet that not only conceals his identity but prevents him from getting whiplash when flying at light speed.
(The Ray 1994 #18 / The Ray 1992 #3)
Somehow Joshua is never given that courtesy--surprising, considering his extreme youth. It's already questionable making your less-than-eight-year-old child your sidekick, but leaving him vulnerable to potential injury and allowing him to be recognizable while you yourself maintain a secret identity...not sure what the thought process is there. There's a lot about Joshua's story that doesn't add up, probably intentionally so.
There is one time he is seen out of costume, but it's not really him, it's in an imaginary sequence in which Ray in a possible future pictures the wedding he wishes he could have had to his childhood best friend Jenny, with his entire family amicably together and in good spirits. Joshua wears a suit and looks like any little boy dressed up for a wedding. This, of course, never happens.
(The Ray 1994 #27)
But it is significant that Ray's fantasies include himself and his whole family having a normal civilian life. We never see Joshua again after this series ended and get no acknowledgment of the brothers' relationship either, but I'd like to assume that it involves Ray's helping his brother have an actual childhood as much as possible, in a way that's healing for both of them.
Starting with not having to wear a cape all the time.