Duarte Galactosemia and diet

My child has a Duarte Galactosemia  (Dg 25%). He is 4 months old, breastfed since his birth. I live in Switzerland where a gen. physician told us it was OK to continue with breastfeeding. I am worried because :
1) Many websites mention a special diet, even in case of "only" Duarte Galactosemia.
2) The weight curve of my son is weird, he has overweight: he has almost 8 kilograms which should be the weight of a 6 month child (he is only 4 month old).

Is something wrong? Should we stop breastfeeding and start soy diet?
Thank you in advance.

[hatbox121]

Here in the U.S. the "experts" are divided on the soy vs. breastmilk or milk based formulas. When my daughter was diagnosed, she was placed on a special diet for the first yr and retested but by the time I had my son(4 yrs later), they had changed their minds and would allow all foods had he tested positive for D/G. I say it's up to you. As far as the weight......he's probably just a chunky baby but you can talk to your dr again if the weight gain continues above normal healthy levels on the charts.