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You husband is only considered a US resident for US tax purpose if you file jointly.  It has nothing to do with immigration (just want to make sure you understand that).  If you want to keep filing jointly, he would declare his worldwide income but he can use foreign tax credit to help offset any double taxation.  Belgium has high taxes so most of his income tax should be offset.  I am not sure which election you both made.

The election under IRC § 6013(h) affords a nonresident alien who is married to a U.S. citizen or resident alien, and who becomes a U.S. resident by the end of the tax year, the ability to be treated as a U.S. resident.

If the election is made, neither of the two individuals making the election can make the election for any subsequent taxable year. Thus, neither spouse may, upon remarriage, ever make an IRC § 6013(h) election with a new spouse.

:A nonresident alien who does not become a U.S. resident during the year in question and is married to a U.S. citizen or resident alien at the end of the taxable year, may elect with his or her spouse to file a joint Form 1040 and be treated as a U.S. resident for income tax purposes for that year under IRC § 6013(g).

Once made, the election is in effect for all subsequent years except for years for which the election is suspended or terminated.  The election is suspended for a taxable year if during that taxable year neither spouse is a U.S. citizen or resident. The election can be terminated at the earliest of either of the following:  Revocation made by either spouse by filing a statement of revocation on or before the last day on which the return for that year is due,  The death of either spouse, (such termination would only affect tax years after the year in which the spouse died),  Legal separation under a decree of divorce or separate maintenance (as of the beginning of the taxable year in which the divorce or separation occurs.