Glad to hear you're supportive I'm bi but didn't have the courage to tell my parents as a teenager, so well done to your son!
Obvious one, but make sure he has access to reliable sex education about gay relationships including protection and STDs- it's often taught very badly or not taught at all in schools.
You might want to see if there are youth clubs in your area for LGBT teens - being gay or bi can be really lonely if none of your friends are. Also be aware that it's very common for gay teens to get bullied. Think about whether he has access to positive role models - things are rapidly improving on this front, but when i was young virtually no tv characters or characters in my books were gay, and gay people mostly appeared as the butt of jokes, which made accepting my sexuality a lot more difficult.
Treat boyfriends the same as you would girlfriends.
Other than that, treat him just the same - he's still the same guy he always was
Sparkly, you are a great mum who will find the words and the empathy to help your ds1 find his way in the world
Like qkate, I also feel that you should set your rules now for ds1 and your younger three children, ie if you do not want ds1 to have a boyfriend or girlfriend stay the night then say so, and keep your rules for your other 3 when the time comes.
(My family are mormons, but I am not) will he be accepted within the temple? and finally what a brave boy to have the strength to come out to you at such a young age. It is truly a testament to you and ds1 that your connection is so strong.
First of all, it's great that he felt able to tell you and that you don't have a problem with it That says to me that you will have no problem in supporting him. Otherwise, I'd just say treat him as you would any other 14y.o. with regards girls/boys. Give him advice if he asks for it. Make your rules (if you have any) known and make them standardised re gender, eg. if a girlfriend can't stay the night neither could a boyfriend or whatever. Just like, show him that you would treat him and a partner of any gender the same way. Also, though bisexuality is a legitimate identity and your son may well be bisexual, it is the case that some young people come out as bi as a sort of halfway house to coming out as gay. At least, that's what I did and so did my first girlfriend and my best friend! And I did find it sort of embarrassing when I had the "So you're a lesbian? I thought you were bi. What about the ex-boyfriend?" conversation with my mother, as I somehow thought she would have been more intuitive. Hope this wasn't too rambling and it was somewhat helpful.