Yes, yeast infection would be showing symptoms in the diaper area. Red sores, that possibly bleed and/or a clumpy white discharge.
If it is in fact a yeast infection, the kid should be serves a very low sugar diet (including fruit and juice), as the yeast thrives on sugar.
But I also would be concerned about a urinary tract infection if it is really strong smelling urine. These types of infections can be serious if the infection spreads to the bladder.
I wouldn't be letting the dad brush off this problem either way, you need to talk about what they have done in the past to fix the problem so you can work together to clear it up. I have personally dealt with both these issues and they don't just clear up without special treatment.