If you are a bonafide resident of Gujarat, then your son will be eligible even if he has studied in another state.

The other clause given in the brochure is 

A candidate who has passed the Qualifying Examination from any other State and, 

(i) has appeared in GUJCET conducted in the corresponding academic year; and

(ii) whose parents are serving in the category of services as shown below and who are transferred from other States to Gujarat and have resumed their duties in the place where they are transferred in Gujarat and shall remain so transferred in the State of Gujarat at the time of registration for admission, shall be eligible for admission and his candidature shall be included in the merit list .

List of Category of Services:

(a) Officers or Employees of Central Government; or

(b) Officers or Employees of Public Sector Undertakings of Central Government or any State

Government; or

(c) Officers or Employees of nationalized banks; or

(d) Officers or Employees of United Nations, UNICEF, World Health Organization and such other International

Institutions located in Gujarat State; or

(e) Gujarat Cadre Officers of Indian Administrative Service, Indian Police Service or Indian Forest Service

working in Gujarat or working in other States on deputation; or

(f) Officers or Employees of Gujarat Government posted outside Gujarat State for administrative reasons.