I've been gluten free for 15 years, and once you get the hang of it it is pretty easy.
If you go into Tesco, Asda, Sainsburys, Morrisons or Waitrose, you'll find a Free From aisle with pasta, cereals (note that normal oats are not going to be suitable as they are normally heavily contaminated with wheat, but ones lablelled gf are OK), biscuits, cakes and muffins.
Most chain restaurants now have gf/wf options, and theres gf cake in all the coffee chains.
The main thing to remember is all the things he can have - meat, eggs, dairy, veg, fruit, rice, potatos. And all packaged food has to disclose gluten, so its easy to find wheat/gluten.
At first it will feel very laborious reading all the labels, but it will become natural after a while.
For picnics I do sushi rolls (V easy to make and can have normal sandwich fillings) or the Warburtons gf flat wraps