asianautica, how would the interest in the addition $400k mortgage @7% be about $1300/month after tax deduction?
Let’s take the tax deduction out of picture first, the interest alone will be over $2300/month (for $400k mortgage @ 7%).
To make your $1300/month after tax deduction, he would need to get tax deduction of $1000/month, it’d be a 43% tax bracket!!!
Giving he already has a loan which enjoys the tax deduction, the additional $400k mortgage will usually not fall into the highest tax bracket.