Yes, Luckygirl, I have an only son who is now in his thirties and we have always been very close. We text (mostly, as youngsters tend not to phone that much anymore) at least two or three times a week, we see each other as often as possible, go on holiday together most years, and generally really just enjoy each other's company.
And no, he is not some single unmarried mummy's boy, but a career police officer with a wife, two great kids and a third on the way, and my DIL is a lovely girl, we have not had a cross word in all the 10+ years we have known each other.
If I had to make suggestions on how to achieve this I would say, give advice when they ask for it, otherwise think your own thing but don't say it. Sure, they'll make mistakes and you might have done things differently with years of experience, but we were that age once and would not have appreciated our parents trying to run our adult lives. Oh, and have a life of your own, so you don't depend on them being the centre of everything.
I would not swap my son for a dozen daughters, I love him to bits and am totally proud of him and how he lives his life 