It's impossible to say what the cause of the pain your boyfriend is experiencing as you have not mentioned the method of removal or how long ago it was done. If a sliver is protruding from the skin removal is extremely easy, with a sterilised pair of tweezers. If the sliver is completely embedded, with no protrusion use a sterilized needle. If the sliver is deep inside its best not to use a needle or tweezers as this could result in bruising to the surrounding tissue, which is one possible reason for lingering pain. Instead you should use a sterilized blade to make an incision and then gently parting the skin, to pick out the sliver.
Irrespective of the method of removal it is essential that you clean the area thoroughly with a disinfectant. Puncture wounds that are laced with lingering bacteria can be easily infected and become a bigger problem. Once you have washed the area you can dab on some antiseptic to further clean the area and speed the healing. Failure to have followed these measures after removal could be another cause of the pain. If the wound does not heal and there is lingering pain as in this case it is possible that there is an infection or there is still a art of the sliver lodged in. The treatment off course would depend on the cause of the pain; whether from infection or the presence of a foreign body. You would need to visit a doctor to have the area examined thoroughly.
answered by G M