I was always told that your entire income is taken into account. I am a father who has just gone through dealing with Child Maintance for the past 9 months trying to get them to agree that my wages are what they are (despite having proof given to them on day 1). If your ex receives income via any means, it should be subject to tax etc and therefore taken into consideration with CMA calculations. To be quite honest with you, CMA/CSA (or whatever they're called this year) are very much always biased on the side of the mother so I would be verrrry surprised to hear that he has managed to get out of paying just because his income is through rentals. I mean it may be that the CSA from whenever it was you started dealing with them had some rules stating that rental is exempt, wasn't it the case some years back that income from rentals wasn't subject to tax anyway, so maybe it is relating to that?
also the fact that he is getting income from renting houses, how many houses and how much of it is actual profit? do you know how much mortgage etc he has? it is only the profit on top of all of the expenditure & interest portion of a mortgage payment that is considered subject to tax. So depending on how he runs his accounts, it could be that it is seen that he isn't turning any profit?