There is little doubt in my mind that any excessive money garnered illegitimatly, i.e. through protection rackets, would have to be laundered. As such he used Genco. If the business was run by his old friend (who's family already had the capital on paper to legitimize the existence and a business like that) it would not draw attention to him. By taking a diminished role he would A) do a favour to his old friend (as GF II pointed out, he felt indebted to Genco's father) and B) Allow for a perfect money laundering front.