I firmly believe that you should never ignore a crying baby.
If your son simply needs assurance that you are there, go in and lean over his cot so he can smell you, and rub his back gently, or his tummy if he is on his back. The combination of your smell and touch will reassure him that you are there. Try to avoid speaking to him if you can, unless you murmer very gently and quietly.
Make sure there is a night-light on, so if he opens his eyes, he can see you.
These simple remedies should help to reassure him, and he will grow out of this - they all do.