Relax your no-play-outside rule. Exercise and excitement both stimulate the bladder and bowels and therefore OUTSIDE is exactly where play should be happening right now! I THINK I can see why you dont let him play outside... but maybe you could take him out on his lead, wait for him to go, say the prompt AS he goes (once only) then as soon as he is done give him a treat, praise and take the lead off. Encourage him to run and play for a little while... it becomes an extra reward for toileting outside. It means he gets chance to play and if he "goes" again, it doesnt matter. In fact, you can give him another reward for it.
When he is inside, encourage him to be sedate. Pet or massage him, train, offer him a chew or just chill out together.
Rather than waiting for him to "squat" and then scooping him up, watch him closely and try to spot earlier signs (circling, pacing, sniffing, returning to a place he has used before)
Keep in mind anxiety also lessens his control, so don't let him see you upset by his mistakes.... if he becomes neurotic about toileting it will be even harder. He may also be stimulated to go after or during a long period of chewing.
What do you do when he makes a mistake? What do you clean up with? Have you tried "moving" the accident to the right place and then letting him see it? With wees you could soak it up with a paper towel or something and then lay that in the toilet area.
For a dog his age and size, I would be taking out at LEAST every 30 mins PLUS at the times you mention PLUS any time he looks suspiscious.... I think you are right, his bladder is tiny and he CAN'T hold it very long.
"Give a man a fire and he's warm for a day, but set fire to him and he's warm for the rest of his life." (Pratchett, Jingo)
"I used to look at [my dog] Smokey and think, 'If you were a little smarter you could tell me what you were thinking,' and he'd look at me like he was saying, 'If you were a little smarter, I wouldn't have to.'" - Fred Jungclaus