without having a clerer understanding of what it actully is he does, it's difficult to make a meaningful comment.
However, if taking arl along is making the work esier for your husbnd, i can't see how it's not paid work.As an example, if your husband was a landscape gardener, nd tking carl with him mde the job quicker, or you husband had to do less of the lumping around, he would pay him i presume?
How old is carl? is it regular work? If your husband pays him, won't he become the employer, with all that brings?