It's not a red flag. He could have flown over to Thailand and had a different woman every night for £20 a time, or paid them £40 a day to be his 'girlfriend' while he was there. That would be a red flag.
I think once people get past the usual age of losing their virginity - between late teens and mid twenties, they start to feel there is a huge issue about it, and it makes them so self-conscious that they think there is something wrong with them.
If you are a good friend, trying to help him, be honest about how he could make himself more attractive to women. If he works out - not to have a six pack - just to be fit and strong with good posture, that would be a good start. Help him choose clothes that women find appealing. Ask what sort of women he's attracted to and help him dress in a way that appeals to that type. Most of all, encourage him to truly believe that women - maybe more than men – find all sorts of men attractive. Not all women do, but some women will. Some like small, skinny rockstar types, or stooping, angular nerdy bookish types, or big bear-shaped men. His height and size don't matter if he is fit and has good hygiene. What matters massively is that he becomes truly confident in himself and his physicality. That will make him attractive. Help him understand he doesn't have to be attractive to every woman - just one or two who he also likes.
I'm giving this advice because DS is not conventionally good looking and got a real hang up about it. I had to build up his confidence a lot. Once he clicked that self confidence and true belief that he could attract the right women would help him, he got lucky overnight. Confidence is everything.