There was no such thing as a prisoner of war for the Romans, at least not in the modern sense. Your hypothetical dude would either be let go to rebuild his life as best as he could, or he would be rounded up and enslaved. We don't know exactly how this process worked, but the general assumption is that "companies" of slavers followed the legions around or had contracts attached to specific provinces. There were equestrian "companies" for nearly every aspect of economic life, so it would be strange if this was somehow the exception. It is commonly assumed that the vast majority of Roman slaves of European origin came via this process.

As he himself was rounded up, if he had any family or dependents, they would also be rounded up. They might be kept together or might be separated; at this point none of them have any rights or considerations whatsoever. It's important not to sugar-coat this, and the brutality of existence in the ancient world is often forgotten. Next they would be sent to local, regional, or super-regional clearinghouses according to their value, skills, etc. If our hypothetical male is in relatively good health, he might be destined for life as a servus rusticus, a farm labourer, or perhaps to the cities. Large males had use as door guards and the like. If he shows fighting potential, of course, he might be destined for gladiator training. A very very very small percentage of male slaves wound up in the gladiator schools, despite the common tropes. Most wound up on middling to small farm holdings as general-purpose helpers, tens of thousands of which were scattered across the Roman Mediterranean by the Imperial period. Many found new (slave) partners there, and had new (slave) children, and just had to forget their old life as best as they could.

Alternatively, if our fellow was captured as an enemy combatant in a major battle worth remembering, he might be kept as a captive destined for the Roman spectacle of the triumph. There, in a great parade, the Roman general would process through the capital in his chariot, and with him "floats" depicting his wartime exploits, including live captives in their exotic get-ups and weapons. This was the fate of the great Gallic chief Vercingetorix, who had to wait years before Caesar finally got around to having his parade (the civil war had to be won first). Vercingetorix was then strangled after the fun.

Contrary to popular belief, enslavement of defeated enemies was not the default during Roman campaigns. It did happen, and in particular as a punitive measure for uncooperative cities under siege and such, but in general the SPQR would much prefer those defeated foes to become integrated into the Roman system, to go back to their lives as producers, and pay in to the economy as provincial subjects. In general, those most vulnerable to enslavement were the ones who found themselves outside typical bounds: without family, without clan or tribe, without citizenship, without protection or advocacy.

As to assimilation: slaves in the ancient world had no choice but to assimilate, and to assimilate fast. Those who did not assimilate did not survive, or wound up in the most unpleasant places (like mines or industrial farms).