"A little nose,lots of hair and looking like his mummy @ChloeSGill our baby boy was born 19/09/14 weighing 6lb15 😍😊 #supermummy #prouddaddy x" the singer wrote on the social media account.
The 27-year-old and his wife Chloe shared the news that they were expecting only at the end of May.
Aston Merrygold, who was in JLS with JB, retweeted the announcement and added: "@JBGill @ChloeSGill proud uncle 🙌 ❤️#thegills"
Earlier in May the couple spoke to HELLO! magazine about getting married in a traditional ceremony on the Isle of Bute – and gave an exclusive interview about their big day to accompany some stunning pictures.
Speaking about why they chose a church ceremony, JB told the magazine: "Chloe and I are both traditional in our views and our values.
"For me, marriage is the union of a man and a woman before God, so it meant a lot to us to have a proper church service with hymns."
The pair dated for six years after meeting on the X Factor where JLS first found fame and officially moved in together after their ceremony, having lived apart prior to the wedding.