D.M.
H.,
Yes, get a nightlight BUT take your son and let him pick them out. I took my son shopping when he started having 'dark at night' issues, and we bought flashlights, nightlights and glow in the dark universe shapes. Then, we had a party at night before dinner and he showed me where to put the nightlights and flashlights and we put the flow in the dark shapes all over the place. Very fun! We came up with a song and dance to chase away the dark, and then at night before bed we'd do a 'dark check' for shadows and such...this was SO awesome and fun for both of us. After a couple of weeks, he stopped wanting to do the check totally but, some night he still does.
Just make it fun, and make it about him! I doubt this is some kind of ploy to get into your bed, but all the same keep him in charge of this and he'll feel like he's big boy.
As for the baby, just make sure he knows how special it is to be a big brother and that he's the best big brother ever! Keep him involved and he's less likely to act out or do silly things to get your attention away from the baby.
Good Luck!