Kill weeds not clover
Unless the weeds are all from a grass family (crabgrass, goosegrass, etc), there is not a chemical that will kill them and leave clover. The chemicals to treat lawns are broadleaf killers, meaning they kill any plant with a true leaf and do not harm grasses. Clovers are a broadleaf and would be killed by these chemicals such as 2,4-D and dicamba.
If he wants a lawn with just grass and clover, he would have to treat for the weeds, and then once the lawn is weed free, over-seed with clover.
thanks for submitting and hopefully this is helpful!