Line Drawing

Any advice before I tattoo my sons portrait? I have always struggled to do hair any tips would be much appreciated.

Yes. Don't.
Unless of course, you're fine with wearing your son's poorly tatted face for the rest of your life on yourself.
Seriously, portraits are not something to start out with, and seeing as you haven't really posted any works you've done, I'll just assume you don't have much experience. So for your own sake, don't rush it, don't fuck it up on your own leg.
If you want to start doing realism shit, start with flowers, or other realistic objects. See how you can do those, then try a face one day.
But if you're stubborn, here's my 2 cents. Take the picture you have of your son and adjust the contrast in photoshop to add some very dark to very light shades. This way it won't look like just another gray splat.

How long you tattoing ?
I suggest if you not have enought tattoo experience dont try make portrait of son or any other portrait.
If that's the best of what you can draw in paper with pencil dont try with tattoo doing portrait.You not make shadow in paper all conture is veri bold.

I agree with the others,portraits are hard to get right and not a tattoo to start out with.id do a load more practise before attempting this.also find a video on YouTube on portrait/realism b&g.it will show you how to build up a portrait and do hair etc.

Some good advise here! tattooing isn't like picking up a pencil. You don't get to erase mistakes and try again...start with small simple designs and understand your tools and what they are capable of doing.....

why not draw it a few times first and send the drawings in, this will give you a better understanding of what you are going to do, if your drawings are not to good it will do no harm and we can advise you on your art work, this is something i would highly recommend before try to do a tattoo portrait, one small mistake and it will not look at all like your son.