The setting is a sci-fi universe (hence the fictional painkiller) but apart from that, has approximately late-80s American technology and resources. It is probably slightly closer to modern day 2011 by medical standards, but ordinary non-Made Up Painkiller medications haven't changed much.
Would he still experience withdrawal symptoms as if in withdrawal for that specific drug, switching from one opioid to another, or would the new medication pick up that slack? (If it varies from drug to drug, since I'm having him coming off a fictional drug anyway I'm willing to bullshit this.) He'd do his own research on the medical aspects of switching over, but probably after the fact as he's in a bit of a hurry. What symptoms would he likely experience? He's been on the previous drug for a substantial period of time (six months to a year). He initially began taking painkillers for chronic post-surgical pain (which makes me suspect he may be in fact switching back to taking something like Vicodin, taking that immediately after the surgery) but switched to Made-Up Painkiller for reasons of availability and necessity. I'm presuming at this point that he's physically dependent on the former drug, but the psychological switch is not a strong element in this case.
Googled: "opiate withdrawal", "opiate addiction", "painkiller addiction", "switching painkillers", "hydrocodone/paracetamol"; Wikipedia research done on opiates, and general analgesics, hydrocodone/paracetamol, as well as morphine and heroin, to which Made-Up Painkiller has been compared.