Belenos is mentioned by a few classical writers, below are the literary references we have of him.

If report does not lie, you were sprung from the stock of the Druids of Bayeux, and traced your hallowed line from the temple of Belenus ; and hence the names borne by your family : you are called Patera ; so the mystic
votaries call the servants of Apollo. Your father and your brother were named after Phoebus, and your own son after Delphi. In that age there was none who had such knowledge as you, such swift and rolling eloquence. Sound in memory as in learning, you had the gift of clear expression cast in sonorous and well-chosen phrase ; your wit was chastened and without a spice of bitterness : sparing of food and wine, cheerful, modest, comely in per-son, even in age you were as an eagle or a steed grown old.

Decimius Magnus Ausonius 310 CE – 395 CE
Commemoratio, IV and X
Written around
Translation by: Hugh G. Evelyn White, The Loeb Classical Library, 1919

Crispinus, it is said, persevered so strongly in pursuing the war which had begun only because there were in the city a great number of aruspices, of men able to read the entrails of the victims, and they gave the more favorable omens. Italians have the greatest confidence in these mysterious consultations. In addition, some oracles were published announcing that the God of the Fatherland promised victory. They call this god Belenos, honor it with an almost fanatic worship, and claim that this divinity is none other than Apollo. Some of Maximin’s soldiers claimed that the image of this god appeared to them in the air, fighting in the defense of the city. I cannot say if they had really seen this apparition, or if they supposed it, so that such a large army did not have to be ashamed of not having been able to resist a troop of citizens much smaller in number, and that it appeared to have been conquered by the gods and not by men.

Herodian 170 CE – 240 CE
Roman history from the death of Marcus Aurelius to the advent of Gordian , III
Translation by: L. Havely, 1871, Paris, Firmin-Didot

So as the siege of Aquileia continued without result, Maximin sent ambassadors to the city. The people would have almost accepted their proposals if Menophilus and his colleague had not opposed them, alleging that even the god Bellenus had affirmed through the intermediary of the haruspices that Maximin had to be defeated. This is why Maximin’s soldiers then claimed, it is said, that Apollo had fought against them and that the victory was in fact due not to Maximus and the Senate, but to the gods. According to some, however, they would have imagined this fable because they were ashamed, they who were armed, to have been defeated by people who were almost unarmed.

Julius Capitolinus
Vie des deux Maximin , XXII, 1-3 in Histoire Auguste
Translation by: André Chastagnol, 1994, Paris, Laffont

Each province, each city also has its own god; thus Syria has its Astargatis, […], Norique its Belenos […]

Tertullian 155 CE – 220 CE
Apologétique , XXIV, 8,
Translation by: J.-P. Waltzing, Paris, Les Belles Lettres