Leweck was close to retiring from football after being diagnosed with a serious fault with a heart valve. He has returned to the game, although his condition cost him the chance to go on a trial with German Bundesliga club Borussia Mönchengladbach.

On the 13 October 2007, Fons scored, in his first game for the national team after a very long injury break, the 1-0 at the 94th minute in the Euro 2008 qualifying game against Belarus. This 1-0 result was the first victory for the Luxembourg national football team in a European Football Championship qualification since 1995. Fons also scored the winning goal against Switzerland in the 86th minute of their second game in the qualifyings for the 2010 FIFA World Cup. Luxembourg won the game 2-1.

He is the elder brother of Charles Leweck, who also plays in midfield for Ettelbruck and Luxembourg.