Yes, since the law changed (2006 maybe?) most 401(k)s have changed their rules to allow IRAs to be rolled into them.
That said - before doing this, you should be sure that you are happy with the investment choices and any fees that his 401(k) charges.
Yes, that’s correct. Most 401(k)s require the rollover to be cash, so he will likely have to cash out all of his investments. He should request a direct transfer from the IRA(s) to the 401(k) of all money except for the basis amount, plus any additional amount he may want to convert. Once the money has left the IRA to go into the 401(k), he can then do a conversion into a Roth IRA.
No need to worry about percentages. The dollar amount of the basis is what is important. He will likely need to sign something for the 401(k) administrator certifying that the money being rolled in is all pre-tax.
Edited to add: If he were to roll some of the basis into the 401(k), he would effectively lose that basis, as all the money in the 401(k) will be treated as pre-tax after it’s rolled in, and he will end up paying taxes on it twice.
As long as you wait until the basis is the only money that’s left in both IRAs, you should be fine to convert the basis into a Roth IRA without incurring any taxes. If, because of timing, the basis earns any interest or dividends after the rest of the money is rolled over into the 401(k), you will be taxed on that.
AJ