Most of what I've written below was on another post which asked a similar question, so apologies for repeating myself.
I recently walked the Portugese camino with my 12 year old daughter, who walked far longer each day than I thought she would. We had a great experience and are planning to walk the Primitivo together next year. A few tips from our trip.
Involve your son in the planning of each day, then they are involved in the trip and quickly get to know what to expect. My daughter (Faith) hates getting up early, but soon realised that this was preferable to walking in the heat of the day, so agreed to early starts. Keep him involved in the process, Faith looked for yellow arrows, and counted them as we went along, posing for photos at key numbers. She counted 1350 arrows between Porto and the Spanish border. Get him to collect the stamps on your credencials, this will create an interest in your progress.
Make friends with fellow walkers. The best part of this was the camaraderie and friendship, we met some lovely people and had great times together. We shared meals at alberges and Faith got a lot of encouragement from fellow pilgrims, she would often walk with other people for short parts of the day. It was April the 1st, so Faith put a clothes peg on my hat brim without my knowledge and giggled every time someone noticed it. This became a joke with our camino friends, and we all took to pranking each other by putting clothes pegs on each others packs.
Keep well fueled with drinks, ice cream and chocolate, this is great for morale, and allows for rests when tired or needing a break. Agreeing to stop at the next cafe is good incentive to push on over the next hill.
Get him a camera, so he can record his trip. Encourage him to pick up a few words of Spanish, and use them, Faith really liked learning new words, and would write down new vocabulary each day. If she saw a sign, she would write it down and google it that evening so she knew what it meant.
Don't be nervous of alberges, the opportunities for meeting other people are great, and he has the chance to make friends with them.
Depending on how you both feel about electronics, an MP3 player with music on it, to help with the long slogs. Faith took her phone with her and send photos to her friends via Instagram and Snapchat, which really helped her motivation and also let her keep in contact with her pals. This also gave her something that was truly hers on the camino, I am an IT dinosaur so this let her have something that was original to her.
It sounds obvious, but talk to each other... Faith told me lots of details of her school life, her friends, her pleasures, etc., the kind of details we don't get time to share during our normal daily routine. We also played lots of games such as "tell me a secret about that pilgrim we met last night", which lets you make up all kinds of fictitious nonsense.
Encourage a desire to make it to Santiago, then celebrate when you get there.
Best of all, enjoy yourselves!