Both statements are true.
He can digest raw sugar and raw honey just fine. He cannot digest the synthetic, man-made sugars very well at all.
Splenda is indigestible, that's why it is calorie free.
Splenda, even though it is calorie free raises his blood sugar level and keeps it raised. Raw sugar and raw honey, also raise his blood sugar levels, BUT not as long, and his body is able to return to normal blood sugar levels after having a food with raw honey / raw sugar as opposed to the synthetics sugars.