Now, Bob is returning to the same theatre. But this time he's playing the lead.
Bob, who grew up in Crookston in the South Side of Glasgow, admits he's delighted to be returning to Siam.
"The first time around was in 1981," he recalls. "My dad, who sang in a wedding band, was also in the production by Pantheon Theatre Club and they were looking for children to appear. So my brother John and myself were roped in.
"John however didn't fancy it. He was into football. But I thought I'd have a go."
Bob had already shown an interest in acting and musical theatre.
"I'd done bits of roles at school thanks to Mr Hunter the history teacher, who organised the school shows.
"And I appeared in productions such as My Fair Lady, The Pyjama Game and Carousel.
"And I liked musical theatre. It was always fun, playing in little halls around Glasgow such as Partick Burgh Hall.
"But when I got the role as a young boy in The King it was a step up to do a really a big show.
"I remember being in dress rehearsals and looking out at all the 1800 empty seats thinking they'd soon be full of people.
"And it was quite terrifying."
But wonderful. Bob loved his original King and I appearance.
But although he was a powerful singer, he didn't see himself going into showbiz full-time.
"I work as a photographer," he explains. "I do mostly portraiture, and there's a showbiz theme because I do headshots of writers, actors, and production photography for shows.
"But I always saw professional acting as a tough life.
"And I never had the hunger for it. It's great for fun. But after the show I'm ready to say goodbye."
He adds, grinning: "A wee bit of adulation is enough every now and then."
Bob gave up his musical theatre hobby when in his 20s. "I suddenly thought it wasn't cool any more. But I'd found my voice and I sang in a band, an originals rock/pop band."
He adds, smiling: "The music was a little closer to REM and Radiohead than Carousel.
"However, in my 30s the smell of the greasepaint became too strong and I went back to musical theatre."
The singer has appeared in a huge range of roles over the years, his favourite being Harry The Horse in the classic stage show, Guys and Dolls.
But his King of Siam role, made famous in the film version by Yul Brynner, will be his first time playing the lead.
"I've finally arrived at the age of 46," he says, grinning.
"And it's great because I'm probably a bit too old for the romantic leads such as Curly in Oklahoma.
"I usually play the dad or the boss or the comedy character. The King is a role the older man can play."
It's especially good for Bob, given he already knows all the songs and most of the lines.
"Back in 1981, I only had one line to perform 'Walk on water?'
"In fact, the other day I was speaking to the little boy Louis Brown who has my line in this production. I said to him 'I'm expecting you to follow in my footsteps and go on to play the King 30 years from now.'
"He laughed and said he'd do his best."
Bob's return to the King, which features songs such as Hello Young Lovers, Shall We Dance and Something Wonderful, is already a success story.
"I've just heard we've sold out every seat for the Saturday matinee, right up to the gods," he says, smiling.
"It's looking good for the rest of the week."
And as soon as the show is over, Bob won't rest on his laurels. He'll be going up auditions for Hairspray, with his am-dram outfit being the first in Scotland to secure the rights to the comedy musical.
"I'd love to get the role of the Mum or the Dad," he says. "But who knows? And just because I've played the King of Siam doesn't guarantee anything."
n The King and I, The King's Theatre, March 4-8.