Yes, it's true. White boxers are white (acutally colourless) because of lack of pigment. And very much as is the case for people, those with very pale skin/little pigment are more prone to burning - and certain consequences of burning, such as freckles and skin cancers - than those with higher amounts of pigment.
The same is true of the dog - where he is unpigmented (and this is more extremely unpigmented than pale-skinned humans) he is more at risk of burning in the sun than where he may be pigmented (or than a normally pigmented dog).
Anyway - what you can do about it is exactly what you've thought of. Use sunscreen

Especially on areas where he may also have very little hair, such as parts of the head. Sticks and sprays are generally better than lotions (last longer, and harder to lick off). He can wear a t-shirt too, for protecting his body in times of longer exposure.