parameterized output valus with dynamic query

declare @sql nvarchar(4000), @tablename nvarchar(100), @reccount int , @param_definition nvarchar(4000) , @param_value int
set @tablename  = 'LA4-PRD-WWW103_W3SVC2_ex120917'

set @sql = 'select @reccount_out = COUNT(*) from ['+@tablename+']'
set @param_definition = N'@reccount_out int OUTPUT';
print @sql

exec sp_executesql @sql , @param_definition, @reccount_out = @reccount output;
print @reccount