Go for it. It will be easier now than in 6 months with an active baby!
If you still can, opt for a night flight.
Nurse/feed for take off and landing (to ease ear pressure)
As previously said, ask for a bulk head seat with a bassinet. Plan on more outfits/diapers than usually in this lenght of time as flying may make him vomit/poop more. Pajamas work best. Take a second T-shirt for you too just in case the burping/vomit lands on you.
One or 2 new toys to keep him entertain.For his first long flight, at around 4 months, my son loved the Lamaze giraffe. If you have an ipod, you can also upload some family pictures/video to use in case of screaming (for some reason, babies tend to stop screaming with this kind of diversion), as normal ways to calm him (walking or dancing around, rocking... may be harder on a plane)
If you use to carry your baby, bring your carrier. This may comfort him too, if needed.
Also bring baby medicine (like tylenol, teething gel or tablet...) as you never know when your little one may need it. And, yes, some baby are teething at 5 months!
As far as I remember for my children, when they were this young, flights were not a problem at all. They would sleep or enjoy.
Enjoy your trip!