I noticed my bf's testicle to be slightly lopsided. Should he be worried?

Not unless hard lump. Some men have testicles which hang sideways, whereas most men have testicles which hand vertically. Occasionally a man can have one of each. He may have a spermatocele cyst or a hydrocele, either of which could make one testicle appear "lopsided". He needs to have it checked out by a urologist if a hard lump is present as this could be due to tumor which would need to be removed.
Lopsided? Normally, one testicle hangs lower than the other usually the left. If there is some misshapenness or other lumpiness of the testicle, I would have your boyfriend see his doctor for a testicular exam.