You don't say his age, but if her son is under 18, you may file the I-130 now, or after he arrives on the K-4. Make sure that he is under 21 when you file it, however, since 21 and over is a different category with a much longer wait. If he was 18 or over when you married, he may not be able to adjust status from the K-4. If he was under 18 when you married, no problem.