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If it is a revocable trust then it is not considered a separate tax entity because the grantors retain control over the assets and can withdraw assets at any time. For income that would belong to your son, you would furnish him a 1099 (for the respective type of income) and file a copy with the IRS. See page 2 reporting as a nominee/middleman - http://www.irs.gov/pub/irs-pdf/i1099gi.pdf
If you did obtain a separate tax identification number, which is required for an irrevocable trust, then a separate tax return (Form 1041) has to be filed each year with K-1s given to each beneficiary for their portions of the income.
Revising my previous post - Because your son is a grantor in addition to you and your wife, a separate TIN should be obtained. See page 2 and footnote 4 - http://www.irs.gov/pub/irs-pdf/fss4.pdf . Also, instead of filing Form 1041 each year the trust could elect the filing procedure under Optional Method 3 on page 7 - http://www.irs.gov/pub/irs-pdf/i1041.pdf