If he was going to be big enough to play rugby, he was going to be too big to go down a mountain bike course. Zander Fagerson had a decision to make. He was already finding it tough enough to squeeze between the trees as he raced downhill; any bigger and he wouldn't make it.

"Other guys were getting faster and they wanted me to get lighter but the rugby guys wanted me to get heavier, so I had to bite the bullet and started eating," he said. "Everybody says the only reason I was good at it [downhill mountain-biking] is because I was so big and heavy but that was actually a disadvantage.

"If you look at the track, it is rocks and trees that have to be pushed apart. Being a big guy, it was getting tough to dodge in and out. You have to put in a lot and go as fast as possible down a set course. If you watch a race, it is pretty intense."

The decision to stick with rugby was vindicated on the eve of his 20th birthday last week, when he got a phone call from Gavin Scott, the Scotland manager, telling him he had been picked for the national training squad. For a tighthead prop with only a handful of starts for Glasgow Warriors under his belt, that showed remarkable faith in his abilities – so much so that even he found it hard to absorb.

"It didn’t really sink in to the next day. I called mum to say 'I think I am in'," he recalls. "It was a very special birthday. I am happy to be here and I'm working hard to get my first cap. I am quite young for a tighthead prop but I feel I am improving week on week."

To underline the scale of what he has achieved, when Sean Lamont won his first Scotland cap as a wing, Fagerson was just eight years old. "Yes, I watched Sean when I was young," he says. "His hair colour has changed and he has taken off those gloves. It was his birthday too the other week and I think he was reminded of it quite a bit.

"When I was young, I was a massive Jonah Lomu fan. I played his [computer] game a lot. He should have been a prop but he was too fast. Euan Murray too – having him at Glasgow last year was massive for me, I learned a lot off him. For me growing up he was the rock of the Scottish scrum. Being a British & Irish Lion too, he was pretty impressive."

The truly exciting prospect for the youngster is that in a season where he is still eligible for the Under 20s, he may well find himself winning Scotland caps. He is competing with Moray Low, the Exeter tighthead, for the bench slot, but after playing in all Glasgow's European Champions Cup games, including an impressive start against Racing 92, and nine league games, he has made a strong case for himself. Low, 11 years his senior, has played regularly for his club but has started only three from 14 matches.

"It was good to get some game time at Glasgow and to get some feedback," said Fagerson. "I was quite happy with Racing 92 game but I have to prove I can do that week after week. Every week is a different position, a different game. If it goes well, good; if it doesn’t, put in on the back burner and use what you learned in future games.

"It was a boyhood dream of mine to play for Scotland. The coaches don’t tell you if you are [ranked] two or three or one, so I will be training as hard as I can and learning all the calls."