My boyfriends penis has a strong smell and around the tip of his penis it looks like it is dirty is it a disease or just dirty?

Disease or Dirty- it doesn't matter. You should speak with him and strongly suggest he take care of the problem.

It certainly isn't particularly safe as far as infection prevention

It sounds like you are referring to smegma, a substance that sometimes has an unpleasant odor. Smegma can form under the penis foreskin, if the shaft and head (glans) of the penis are not cleaned properly. Smegma is formed by oils produced by the body and bacteria that feed on the oil. It can be easily washed away with mild soap and water.

In its fresh state, smegma is a wholesome and functional lubricant. But, if allowed to accumulate in the foreskin cavity, it becomes changed into an unpleasant, unhealthy, and bad-smelling substance. In addition, foreign material may gain access to the smegma deposit in the form of dirt, dust or sand, and urine and semen.