The short answer is – unfortunately – yes, if the child starts using it less, be it a first, a second, or a third language. “But he speaks it so well, how can he forget it?” Yes, such cases have been described in the literature. And there is an abundance of anecdotal evidence. The degree of forgetting varies a lot.
“I spoke Russian better when I was younger”
“I spoke only Inuktitut until I started school. Now [as an adult] I understand it, but when I try to say something, the words just don’t come out right. So I switch to English”
“My parents say I spoke only French till I was three. I don’t remember that. Then we moved to the U.S., I learned English, and could not speak French until I started taking French classes later.”
“We always spoke Russian to my daughter. When we lived in Israel, she learned Hebrew at the daycare. She was fluent. When we moved to Canada, she went to Senior Kindergarten. We concentrated on her English, continued to speak Russian at home, and there was no place where she could speak Hebrew. And she lost her Hebrew in a few months! I wish I knew it could happen so fast!”
And the most dramatic language loss was found in Korean adoptees in France (studies by C. Pallier, V. Ventureyra and their colleagues). These children lived in Korea until their adoption at age 1-8 – some of them even went to school in Korea, – haven’t heard any Korean since adoption, and completely forgotten it.
Some children go through several periods of forgetting and re-learning when their language situation changes. An Inuit girl who has to spend long periods in hospital outside of her community forgets Inuktitut by the end of each hospital stay, and then re-learns it when coming back home. Children in a mixed German-Russian marriage in Germany forget Russian by the end of the school year, re-learn it during summer in Russia with grandparents, but forget German by the end of the summer, go back to Germany, re-learn German, but forget Russian, and so on.
Modern studies suggest that the age until which a complete language loss or dramatic forgetting can happen is somewhere at 8-10 years old. Although the basic first language knowledge is already in place by about age four, 8-10 is the age when the first language knowledge stabilizes, and also the age when a new language is learned in the same way as by adults. This does not mean that forgetting cannot happen after this age, only that there will be much less forgetting.