From http://psoug.org/reference/roles.html, create a procedure on your database for your user to do it:
CREATE OR REPLACE PROCEDURE GRANT_SELECT(to_user in varchar2) AS
CURSOR ut_cur IS SELECT table_name FROM user_tables;
RetVal NUMBER;
sCursor INT;
sqlstr VARCHAR2(250);
BEGIN
FOR ut_rec IN ut_cur
LOOP
sqlstr := 'GRANT SELECT ON '|| ut_rec.table_name || ' TO ' || to_user;
sCursor := dbms_sql.open_cursor;
dbms_sql.parse(sCursor,sqlstr, dbms_sql.native);
RetVal := dbms_sql.execute(sCursor);
dbms_sql.close_cursor(sCursor);
END LOOP;
END grant_select;