That Tooth which is hanging from the gum of the child is a milk tooth whose time has come to be exfoliated or in simple terms it should have had shed at age of 6 years or 6. 5 yrs. All you need to do is visit a dentist ,so the dentist can take out this over retained tooth. There is nothing to worry as it is completely painless procedure and a new permanent tooth will erupt in a short time after this tooth is taken out.
Generally the front anterior teeth shed on their own between the age of 6 to 7 years. Since your son is 7 years old and still the permanent interiors have not erupted in the oral cavity ,the best option would be to visit a dentist so the dentist can take an x-ray and see the exact position of the teeth. There is nothing to be worried about ,just visit a dentist and get the examination done so that the position of the permanent incisors can be evaluated and if any treatment is required regarding eruption of permanent incisors then you will be guided accordingly.
Kindly get an X-ray done of the broken tooth. May be you will have to get root canal treatment done followed by a cap/crown. Brush your teeth twice daily using correct brushing technique to maintain your oral hygiene.
As your doctor said you have to get a replacement since you only have front teeth its best you go for implants or an over denture which is fixed and more comfortable. You can also go for a removable denture but it will not be as comfortable.