My husband contributed $5500 to ROTH in 2018 for 2018. Now he wants to switch to traditional IRA to lower the taxable income. Can he do that in 04/2019 for Tax year 2018?

Yes,  You have until the due date of the tax return (Apr 15) or extended due date (Oct 15 if a timely extension is filed) to recharacterize the Roth contribution to a Traditional IRA contribution.    Ask the IRA custodian to do that.

The proper way to report the recharacterization and earnings which is to enter the 2017 IRA contribution in the IRA contribution interview section and then say yes to "Did you switch from a Roth to a Traditional IRA - recharacterize".

The amount The amount of the original Roth contribution must be entered - not any earnings or losses.

Then TurboTax will ask for an explanation statement where it should be stated that the original $xxx.xx plus $xxx.xx earnings (or loss)  were recharactorized.

If the IRA custodian issues a 2019 1099-R next year with a code R i box 7, it can be ignored if you enter it as above for 2018.