Yes, Michigan can suspend your son's Wisconsin driver's license in their state meaning that his driving privileges would be suspended in MI. Michigan cannot suspend your son's license for all states however his home licensing state of Wisconsin can. When your state of licensing suspends your license it is suspended in all states since other states require a valid license for you to drive in them and yours would not be valid per your licensing state of Wisconsin.

When you commit violations in other states that call for your license to be suspended than when that state suspends your license it is for that particular state only. However since most all states share information your home state will normally be notified of your out of state DUI and also suspend your license.

The Driver and Vehicles section of the Wisconsin Department of Transportation notes regarding out of state violations that no points are assessed for convictions from other states, however, the conviction is entered on the Wisconsin driver record.

They go on to say that Wisconsin takes action on any out-of-state convictions for which Wisconsin statutes require a mandatory suspension or revocation of the operating privilege. Those convictions include:

• Operating while under the influence or with a prohibited alcohol concentration.
• Attempting to elude an officer
• Failure to stop and render aid after an accident resulting in - Death of another, great bodily harm or Personal injury to another. 
• Serious property damage (if specified by the court)
• Injury by negligent operation of a motor vehicle while under the influence
• Great bodily harm resulting from the operation of a motor vehicle while under the influence
• Homicide resulting from the operation of a motor vehicle while under the influence
• Perjury or making a false affidavit or statement to the department
• Reckless driving
• Vehicle used in the commission of a felony

So once your son is convicted of the DUI in Michigan their courts should inform the WI DOT and it appears at that time the state of Wisconsin will suspend his license. The Michigan suspension would be for just the state of MI as their DMV states - even if you hold a valid license from another state, you are not authorized to drive in Michigan if your operating privilege is revoked, suspended or disqualified by the State of Michigan. Then if WI also suspends his license it would be suspended for all states since he would not have a valid license per his licensing state.