You may need to see a tax professional.
Assuming your adjusted cost basis is $295K as you say (but be careful, not all closing costs from the purchase are allowable adjustments to basis, see publication 523). https://www.irs.gov/pub/irs-pdf/p523.pdf
And assuming your total gifts of equity are $194,700 (exactly 2/3, but you need to determine how much it actually is).
Then at the present time, your son owns 2/3 the home and you own 1/3 the home. If you sell for $600K (and assuming a 6% commission) the adjusted selling price is $564K.
But your son owns 2/3. So he reports a capital gain of $372K (2/3 the proceeds) minus $194K (2/3 the basis) equals $178,000. Because he does not live there, he owes full capital gains tax on this gain. Your gain is 1/3 the total, because you own 1/3, and your gain is covered by the $250,000/$500,000 exclusion on gain from a personal residence.
And, if your son lives in a different state, he will need to file a Mass non-resident return to report this income in Mass because it is Mass-source income. He will also report the gain in his home state. His home state should give him an offsetting credit for taxes paid in Mass to reduce the impact of double taxation.
