SQL Server table to json

I wouldn't really advise it, there are much better ways of doing this in the application layer, but the following avoids loops, and is a lot less verbose than your current method:

CREATE PROCEDURE dbo.GetJSON @ObjectName VARCHAR(255), @registries_per_request smallint = null
AS
BEGIN
    IF OBJECT_ID(@ObjectName) IS NULL
        BEGIN
            SELECT Json = '';
            RETURN
        END;

    DECLARE @Top NVARCHAR(20) = CASE WHEN @registries_per_request IS NOT NULL 
                                    THEN 'TOP (' + CAST(@registries_per_request AS NVARCHAR) + ') ' 
                                    ELSE '' 
                                END;

    DECLARE @SQL NVARCHAR(MAX) = N'SELECT ' + @Top + '* INTO ##T ' + 
                                'FROM ' + @ObjectName;

    EXECUTE SP_EXECUTESQL @SQL;

    DECLARE @X NVARCHAR(MAX) = '[' + (SELECT * FROM ##T FOR XML PATH('')) + ']';


    SELECT  @X = REPLACE(@X, '<' + Name + '>', 
                    CASE WHEN ROW_NUMBER() OVER(ORDER BY Column_ID) = 1 THEN '{'
                         ELSE '' END + Name + ':'),
            @X = REPLACE(@X, '</' + Name + '>', ','),
            @X = REPLACE(@X, ',{', '}, {'),
            @X = REPLACE(@X, ',]', '}]')
    FROM    sys.columns
    WHERE   [Object_ID] = OBJECT_ID(@ObjectName)
    ORDER BY Column_ID;

    DROP TABLE ##T;

    SELECT  Json = @X;

END

N.B. I've changed your two part object name (@schema and @table) to just accept the full object name.

Example on SQL Fiddle

The idea is to basically use the XML extension within SQL-Server to turn the table into XML, then just replace the start tags with {ColumnName: and the end tags with ,. It then requires two more replaces to stop add the closing bracket to the last column of each row, and the remove the final , from the JSON string.

Use from the magic words For JSON

example:

 SELECT name, surname  
 FROM emp  
 FOR JSON AUTO  

result:

[{"name": "John"}, {"name": "Jane", "surname": "Doe"}]

more info in:

https://docs.microsoft.com/en-us/sql/relational-databases/json/format-query-results-as-json-with-for-json-sql-server?view=sql-server-2017&viewFallbackFrom=sql-server-2014